Corundum is crystalline aluminum oxide, Al2O3, second
hardest on Mohs Scale, and one of the most popular (and expensive) of gem minerals. Ruby
is
red corundum; all other colors are Sapphire.
Sapphire is
commonly used to denote blue corundum, with others named according to color, e.g. pink
sapphire, yellow sapphire, etc.. The name Star Sapphire is reserved for
asteriated,
blue corundum. Paradoxically, star stones that are not blue are sometimes referred to as
Star
Ruby, even those that are not red!
Padparadscha (pod-pa-Rah-sha; lotus flower)
is an ill-defined name for rare pinkish orange sapphire. The chromophore in ruby is Cr (+
Fe), in blue sapphire Fe and Ti, in yellow sapphire Fe and/or a yellow color center.
Padparadscha is colored by trace impurities of Cr and Fe, with or without a yellow color
center. Some green sapphire contains trace amounts of Ni as a chromophore. Pure corundum
is colorless.
Corundum is a relatively common mineral that
once found wide application as an industrial abrasive. Gem corundum is mined almost
exclusively from gem gravel deposits. These deposits are derived from the weathering of
high temperature metamorphic (marble, gneiss) or igneous (volcanic or pegmatitic) source
terrains. Historically, the most famous and prolific production has been from
Myanmar (ne
Burma), Thailand, India and
Sri Lanka (ne Ceylon). More recent important sources include
Australia and the East African countries of Tanzania and Kenya.
Corundum was the first gem mineral to be
synthesized in the lab by a process known today as flame fusion. A somewhat more
difficult process, flux growth, is also used to synthesize gem corundum. Synthetic
rubies and sapphires are presently manufactured in enormous quantities for both industrial
and gem application. Although no longer widely used as an abrasive (silicon carbide
[H=9.5] and diamond bort or carbonado are used instead), industrial applications today
include use as "jewel" bearings in instruments and fine watches, and as a source
of stimulated emission in ruby lasers. Because of its hardness, clarity and ease of
synthesis, synthetic corundum is also used today for windows in optical scanning devices,
like those in grocery store checkout counters.
The advent of synthetic corundum in the first
decade of this century caused widespread apprehension and greatly diminished ruby sales.
Extensive British ruby and sapphire mining operations in southeast Asia, particularly in
Burma, suffered greatly from the resulting fall in prices and never recovered. Curtailed
mining operations were finally closed after voluntary liquidation in the late 1920's.
Though extensive and rich deposits apparently still remain, ruby production today from
Myanmar is only a fraction of what it was at the turn of the century.
As discussed below, the color and clarity of
rubies and sapphires is commonly enhanced by heat treatment and, less frequently, by
irradiation. Although not commonly disclosed by retailers, such processes are widely
acknowledged within the industry and are probably today the rule rather than the exception.
Reliably distinguishing among treated, untreated, natural and synthetic
ruby and sapphire depends critically on microscopic examination of inclusions
and crystal growth features.
Properties
- Crystal System: Hexagonal
- Habit: Usually as
well-formed hexagonal prisms
with or without rhombohedral terminations. Sapphire often as
elongate prisms; ruby usually
stubby, flat prisms. Waterworn pebbles are often rounded variants of these shapes.
- Hardness: 9
- Cleavage: None; often a basal and rhombohedral parting,
however.
- Toughness: Excellent
- Specific Gravity: 4.00 (3.99-4.02)
- R.I.: Intermediate; 1.761-1.769 (reported range
1.757-1.778)
- Birefringence: Low (0.008-0.009)
- Dispersion: Low (0.018)
- Pleochroism: strong, with deepest color perpendicular to c
axis:
- Ruby - intense purple-red to lighter orange-red
- Blue Sapphire - violet-blue to lighter greenish-blue
- Orange Sapphire - orange to lighter yellow brown or pale orange
- Purple Sapphire - violet to orange
- Yellow Sapphire - yellow to pale yellow
- Green Sapphire - green to blue-green and yellow-green.
- Pink Sapphire - hot pink to pale pink or more red.
- Asterism: 6 (more rarely 12)-rayed star due to oriented
rutile needles.
- U.V. Fluorescence: varies with place of origin (Fe
content);
- Burmese Ruby - strong red (Cr); best stones fluoresce in strong
sunlight.
- Thai Ruby - less intense red (Cr+Fe); may show patchy blue
fluorescence if heat treated.
- Sri Lankan Ruby - strong orange red in long u.v.,less strong in
short u.v. (Cr).
- Pink Sapphire - same as above.
- Padparadscha - same as above. Heat treated may show strong reddish
orange in short and long u.v..
- Green and most yellow Sapphire - none (Fe + Ni + Ti).
- Blue Sapphire - none (most; Fe + Ti) to red or orange (Kashmir,
Sri Lanka, Montana) in long wavelength u.v.. Heat treated stones may show a dull chalky
green fluorescence in short u.v..
- Colorless - moderate light red to orange
Distinguishing Properties
RUBY
- Distinguished from red spinel, garnet or glass (all isotropic)
with polariscope. R.I. for other red stones is sufficiently different to
distinguish.
- From synthetic ruby by microscopic examination or fluorescence in
a few cases.
- Synthetic ruby grown by the flame fusion technique usually shows
curved growth striations or curved color zoning, and may contain unmelted grains and gas
bubbles. Growth stria and color zoning are best observed by examining stones through the
table because of the way synthetics are oriented for cutting; the c-axis, which is
parallel to the long dimension of the boule in flame fusion corundum, lies parallel to the
table of gem stones cut from these boules, whereas natural ruby is often cut with the
c-axis perpendicular to the table.
- Under short wave u.v., flame fusion synthetic rubies often
show a brighter red fluorescence than naturals, though the test is nondiagnostic because
some Cr-rich synthetic varieties are less fluorescent, and the best Burmese gems are as
fluorescent as most synthetics.
- Flux growth synthetic rubies (e.g.
Ramaura
ruby)
may contain flux
inclusions (orange or white in color if PbO or PbF flux is used).
- Flux-growth Ramaura ruby in our collection fluoresces
a strong red in long wavelength u.v. light but shows little to no fluorescence in
short wave u.v.. Some Ramaura ruby apparently also fluoresces
strongly in short wavelength u.v. Fluorescence in both short
and long u.v. is said to be more yellow-orange than that observed in
natural ruby.
- In all cases, synthetic ruby lacks the natural mineral or
fluid inclusions that are
distinctive for ruby from all sources (see below).
SAPPHIRE
- From blue or other spinel by polariscope and R.I., from others by
high S.G. and R.I..
- From synthetic sapphire by inclusions and by same features listed
above. Curved striations and color zoning in flame fusion synthetics are less visible than
in synthetic ruby, but can sometimes be seen if the gem is placed in immersion liquid.
- Under short wavelength u.v. light, synthetic blue sapphires
show a bluish-white or greenish glow, which is only very rarely encountered in natural
sapphire.
- Synthetic purple or violet sapphire resembling amethyst in
color glows bluish-white in short wavelength u.v. light and red in long wavelength
u.v..
- Natural yellow sapphire will sometimes fluoresce in short
u.v. light; synthetic yellows will not.
- Synthetic "Alexandrite-like" color-change sapphire
in which Vanadium is added will show prominent growth lines and a characteristic
purple-mauve color.
Ruby
The king of gems. In sizes over
10 carats, Burmese stones of the highest quality are, per carat, the most
costly of all
gems. Birthstone for July.
Color is of principle importance in pricing. "Burma"
rubies are renowned for an intense, medium, pure to slightly purplish red color, whether
from Burma or not. The ill-defined adjective "pigeon blood" has been used to
describe this color, though its precise meaning today is so obscure that the term is
virtually meaningless. The best, true Burma rubies show a red, warm glow in direct
sunlight, a consequence of strong u.v. fluorescence peculiar to these stones that are
colored by Cr without any Fe present. "Thai" or "Siam"
rubies are commonly darker, with a darker tone and weaker saturation (Fe present), and
comprise most of the stones on the market today. Some or all of these darker
overtones are today commonly removed by heat treatment. "Ceylon"
ruby was once a common term for light red to pinkish ruby that in most cases could more
properly be referred to as pink sapphire. It should be emphasized these names
today have very little to no meaning with regard to a rubies origin (they probably
never did, given the inconsistencies of their use and the broad range of colors produced
at all localities) because of the now widespread use of color enhancement techniques
(see below).
The term "Balas Ruby" is an
old misnomer that was once used for red spinel, and is not ruby at all.
Prior to the mid 1960's, much of the ruby sold
in America originated from
Burma. Discerning jewels and gemologists considered Thai rubies
to be greatly inferior to the lighter, purer red, Burma varieties; these stones were
consequently worth considerably less. Production in Burma nearly ceased after socialists
seized power in 1962, and today accounts for an extremely small portion of fine rubies
that annually make it to market. Thai rubies have since become the norm (average consumers
describe ruby as "dark red") and have experienced a tremendous price increase.
The lighter, purer-red, Burmese varieties are still the most highly
prized, but are rarely
seen in jewelry stores today. The common notion that the best rubies are very dark red
must be dispelled when purchasing fine gem ruby.
Sources
- A. Myanmar (ne Burma) - Worlds
finest rubies from Mogok Stone Tract; since
at least 1597 A.D. (historical records); perhaps as far back as the Neolithic (stone tools
found in the area). Recent production (beginning ca. 1992) from a new locality, Mong
Hsu, belies the superlative "Burma Ruby" moniker. These are not nearly the
quality of Mogok
ruby, commonly requiring heating and filling to yield
viable gems. By far the most definitive work on Burma corundum is that by Hughes, which is now online and makes fascinating reading.
Mogok ruby has the following characteristics:
- Originate in marble formed by contact(?) or regional(?)
metamorphism of impure limestone. Mines are in gem gravel deposits.
- Inclusions of rutile, calcite, apatite, olivine,
sphene, spinel.
- Give off a strong red fluorescence in short and long
u.v. light.
- Most prolific production during the period 1889-1931, when the
area was mined on a large scale by Burma Ruby Mines Ltd., a British concern run by an
eminent London jeweler, Edwin Streeter.
- Mining on a small scale by native miners using primitive methods
from 1931-1963, except during W.W.II.
- Private gem mining officially ceased by the socialist governments
decree in 1962; mines nationalized in 1963.
Mining continues today
through government leases to Myanmar nationals. Some production
is sold at government-sponsored auctions in Rangoon, some changes
hand privately via smuggling
through Thailand.
- B.
Thailand
-
once
about 70% of world production. Important because of lack of commercial mining in Burma.
- Darker red; brownish to purplish overtone due to Fe
- 85-90% from the Chanthaburi-Trat gem field
(since
1850?), 330 km
SE of Bangkok.
- Mining restricted to Thai nationals since 1919.
- In lateritic soils atop Plio-Pliestocene basalt, or in gem gravels
derived from basalt. Deposits are about 6-20' below the surface.
- Source is thought to be basalt (corundum
xenocrystic?).
- More Fe-rich than Burma ruby, show a much weaker red than Burmese
stones in short and long u.v..
- Characteristic inclusion is pyrrhotite (iron sulfide).
- C. Sri Lanka - most of the rest of the worlds production
(since 200 A.D.?).
- Main deposits (gem gravels) in the Ratnapura district, about 100
km S.E. of Colombo, and Elahera district (115 km NE of Colombo).
- Rubies are typically more pinkish (could be
referred to
as pink sapphire in some cases) than those of Burma or Thailand.
- Characterized by sparse, long, rutile inclusions, and included
zircons with radiation halos.
- Give off a strong orange-red in long
u.v. light.
- Deposits have yielded some exceptional star stones (e.g. Rosser
Reeves in Smithsonian).
- D. Cambodia - Pailin Gem Fields
- Cross-border extension of the Thai gem fields; same
characteristics as Thai ruby.
- E. Others -
Vietnam, East Africa (Kenya,
Tanzania; exceptional color, generally fair to poor clarity for material produced prior to
1984, new finds are said to rival Burmese for color and clarity, after heating), Australia
(mostly sapphire),
India
(mostly lower quality stars and cab. material),
Madagascar, Pakistan,
Afghanistan,
Nepal.
Shaping and Treatment
- Step or brilliant cut; heavily flawed or star stones are cut en
cabochon.
- Much ruby rough is shallow; many cut stones are shallow as a
consequence. Because of strong dichroism, the best color is attained by cutting the table
normal to the c-axis. Ruby cut this way will look dark in all orientations when placed
table-down on a polariscope. Stones cut for weight in any other orientation tend to show
more orange in the color than those cut with the table normal to c.
- Heating is used to remove dark brownish or purplish overtones in
"Thai" and other
rubies and lighten the color. According to most reports, this is a now common
practice. Faceted stones that have undergone
such a treatment have to be repolished;
double girdles, pock-marks, or other indication of repolishing sometimes a clue to
treatment. CO2 inclusions will not survive high temperature treatment; their
presence is good indication of no heat treatment. Discoid fracture patterns around natural
mineral inclusions are also a sign of heating.
- Asterism can be induced in stones containing sufficient Ti by
heating for an extended period at about 1300oC (rutile needles form). Can also
be eliminated by heating to higher temperature (1600-1900oC) for short period (rutile dissolves).
- A surface diffusion process is also used to enhance color in
weakly colored material. Can be detected by color concentration along facet junctions, in
surface-reaching fractures and around the girdle.
- Clarity is now commonly enhanced by
"flux-healing", particularly in Mong Hsu ruby. See
Hughes for an
excellent description of this process.
Standardized laboratory grading terminology now includes terms to
describe the extent of such treatment.
Pricing and Valuation
- Priced
on a per carat basis, according to color, clarity and size of stone.
- An excellent discussion of the factors involved in judging quality
can be found in Chapter
10 of Hughes. A table of famous rubies
can also be found there.
- Finest quality, in 10 carat or larger sizes have sold at auction
for over $100,000/carat! ($3,630,000 was paid for a 15.97 ct Burmese ruby in 1988;
that's more than $225,000/ct!). $1,239,000 was paid in 2000 for a
9.98 ct Burmese ruby set in ring with diamonds.
- Finest quality in the 1-2 carat range was selling for about
$6,000/carat (wholesale) in 1990. Price trends (retail) for the same material
over a 25
year period show a recent increase.
- Commercial grade ("Thai" or Mong Hsu;
poorer color, +inclusions, poor cut)
stones of the same size was selling for about $100-1000/ct (wholesale) in 1990, as it is today.
- Fine gemstones of documented Burmese
(Mogok
only) origin sell for a premium. A 3 carat unheated Mogok ruby can sell for as much as
3 times the price of a treated Thai or Burma counterpart. The price difference decreases
in smaller stones (50% premium for 1 carat stones; 25% premium for melee).
- Proper cut and proportions important, as in all stones. Beware of
shallow cuts.
- Ruby made by flame fusion or crystal pulling techniques is
manufactured in enormous quantities and sells for a few dollars a carat. Flux-grown
synthetic rubies manufactured by
J.O.
Crystal Company ("Ramaura Ruby"),
Chatham, and others are considerably more
expensive. They are sold in 4, inclusion-based, grades (Gem, Fine, A, B) and several size
(weight) categories. Sub-carat, Gem grade stones sold for about $200/carat and 5-6 carat
Gem grade stones for $350/carat in 1989 (wholesale). Grade A stones are 1/3 to 1/2 of
these prices. The flux-growth process typically yields quite thin ruby crystals; as a
result, a lot of the stones being offered are quite shallowly cut. Other manufactures of
flux-growth rubies include Gilson, Kashan (now
defunct), Knischka, and Lechleitner.
- Despite disingenuous
claims
to the contrary, the flux growth
process does not more closely resemble natural ruby growth
processes (i.e. solid-state metamorphic processes; gem ruby and sapphire
do not crystallize from magma even though they do occur in volcanic
rocks) than synthesis by flame fusion
or crystal pulling. As for looking "more natural", once faceted
all types of synthetic ruby are, to the naked eye, indistinguishable (see this for
yourself in lab) and both so closely resemble fine natural
ruby that only very careful, microscopic examination reveals a difference. Why
do people pay two orders of magnitude more for
flux-growth synthetic ruby when much less expensive, essentially identical flame
fusion ruby is available? Advertising, lack of knowledge, and
"price guilt" are prime factors; there appear to be no
gemological/mineralogical reasons to do so.
-
Star Ruby
Fine red star rubies are one of the most
expensive of all colored stones. Fine gemstones are exceeding rare, more so now because of
the recent, widespread(?) practice of heating potential star rough to turn it into clear
ruby.
At its best, star ruby has the body color of Burma ruby (no brown, little to no
purple). As in all fine star stones, the star should be centered, with sharp (not wide or
blurry) rays that extend to the base of the cab. The star itself should be silvery or
milky white with no "bleed-through" of red.
Top stones are
translucent.
Prices per carat for the best gems can those of the best Burmese ruby.
Asterism can be (and is) induced in some ruby by
a surface diffusion process. TiO2 powder is painted on a ruby cab., which is
then heated to near melting temperatures. TiO2 diffuses into outer few microns
of the stone. Cooling and subsequent annealing produces aligned rutile needles in a thin
outer layer, resulting in a star.
Sapphire
Birthstone for September. Occurs in a
wide range of colors (blue,
pink, "padparadscha", orange, yellow, green, purple, black, colorless). Color is
due to trace impurities of Fe2+, Fe3+ and Ti, and/or a yellow color
center. The most desirable (i.e. expensive) color is an intense "cornflower"
blue; these are sometimes referred to as
"Kashmir"
sapphires. At their best these have a very saturated, slightly milky, violet blue
color. Padparadscha (see above) is next in value, followed by pink, then orange, purple
and yellow, respectively. Though sometimes touted as a good investment, green sapphires
are not currently, and have not historically been, of great value. The color of most
sapphire can be altered through heat treatment (see below) and/or irradiation.
Sources
- - Major producers same as ruby (Burma, Thailand,
Sri Lanka), plus
Australia and East Africa (Tanzania, Kenya, Nigeria, with lesser amounts from Malawi and
Burimundi). Production in Thailand has seen a dramatic
decrease in recent years. All mined in gem gravels or clay resting on basalt.
Some of the finest
blue sapphire currently being mined comes from the Rakwana district of Sri Lanka.
- Sources of note:
- 1) Kashmir,
India; discovered 1881. Deposit in a remote Himalayan glacial valley where corundum
occurs in association with pegmatite veins in
marble. Found in pockets where plagioclase
altered to clay; could be scooped out by hand. Mines were thought to be essential
depleted(?) by 1925, when mining was greatly curtailed. Later mineral survey revealed much
of the valley floor is underlain by sapphire-rich soil. Has been no official sales of
Kashmir rough for 20 years. Mines not currently active, though locals are said to mine
clandestinely. True Kashmir sapphire rare.
- Color is described as an "unrivalled"
blue; a
quintessential rich, royal, velvety blue with a fine satiny luster.
- All shows color zoning; diagnostic
- Proving a Kashmir origin is difficult at best, impossible in most
cases without papers that document source of origin and history of stone.
- May contain inclusions of green mica, tourmaline, and/or fine
rutile.
- 2) Burma - Mogok stone tract. Considered
second only to Kashmir sapphire.
- Believed to have originated in pegmatites and nepheline-corundum
syenites (not in the marbles that contain ruby); mined from gem gravels.
- Color is exceptionally even; commonly lacks color banding typical
of Sri Lankan blue sapphire, which it otherwise closely resembles. Rich
"intense" blues, verging on violet are considered the best, but lighter blues
are more common. Purple, straw yellow and green are also found.
- 3) Montana
(Missouri River, NE of Helena, also Yogo Gulch, near Utica); discovered 1865, 1895. Also
from Rock Creek in Granite Co.. Early and continued sources of much industrial-grade
material. Also the source of considerable gem material (about $25 million from Yogo Gulch
from 1895-1929).
Gold and sapphires in Missouri river gravels mined intermittently from 1865-present.
Sapphires usually quite small, well-rounded or flat, and of a pale blue or blue-green
color.
- Best
Yogo Gulch blue sapphire are said to rival Kashmir for color,
show better clarity. Majority are a paler blue or have a greenish tint. Some are a lovely
purplish-blue. Color most commonly associated with the term "Montana Sapphire"
is stone of medium to light tone, very transparent, with what has been described as a
steely gray blue or somewhat metallic blue color.
- Rough is typically small (most yield subcarat stones), often
shallow. Rough that will produce a gemstone of 2 carats or more is rare.
- Yogo Gulch mine is in 2-8' wide dike that cuts limestone. Hard
rock mining techniques and American labor makes mining relatively expensive, less economic
than East Asian counterparts.
- Yogo Gulch first mined on a large scale by an English sapphire
syndicate, beginning in 1898. Production nearly halted during W.W.I. Following war, worker
and water shortages, production problems, and double taxation (British and U.S.) led to
closing in 1929. Mine reopened in 1956. Since then no less than 13 attempts have been made
to reestablish Yogo sapphire production. All produced sapphire, none did so economically.
Current (1993) owner and operator is Rancor Inc. (since 1985?) and Vortex mining who are
developing a recently discovered extension of the dike. Geological surveys indicate a
minimum of 28 million carats are still in the ground, making this one of the largest
proven deposits in the world.
- 4) Australia
(Queensland: Anakie and Lava Plains districts;
New South Wales: Glenn Innes and Inverell
districts); discovered 1870, 1960 respectively.
- Gem dirt (alluvium) at the surface, to 50 ft. thick atop weathered
basalt.
- Anakie noted for fine yellows, also some very fine blue but most
has been characterized as "inky" (too dark) or strongly dichroic dark blue and
green; some production of pink and orange as well.
- Dark sapphires are bought by Thai dealers and others
who lighten the color by heat treatment (see below).
- Currently a major producer (50-70% of world production in 1990,
according to one source).
- 5) Tanzania; Umba River Valley near Kenyan border; since 1962.
- sapphire in a "corundum pipe" (dike?, basalt flow?)4
miles in diameter.
- according to one source, the market in "East
African" corundum was cornered in 1965 by a group of dealers; most rough still uncut.
- noted for past production of fine orange-pink (some consider the
brown overtone present in these stones precludes the padparadscha moniker), deep purples,
dark, untreated golden yellows, and color-change (i.e. alexandrite-like) sapphires.
- Also specimen and some gem grade
ruby in green zoisite rock (a.k.a Ruby in Chrome Zoisite) at
Longido
- 6) Others: Pakistan, Cambodia (Pailin Gem Fields), Malawi, Colombia,
Brazil, Nigeria, China, Zimbabwe, Madagascar.
Shaping and Treatment
- Faceted in both step and brilliant cuts. Star stones and included
rough cut en cabochon. - Heat
treatment of sapphire is a widespread practice (nearly ubiquitous?), irradiation less
so. Following treatments and results have been noted:
- Heating in open crucible from 1500-1900oC of pale
yellow or nearly colorless stone can yield a rich dark yellow, golden, golden-brown,
orange, or reddish-brown color (Fe2+ oxidized to Fe3+). Pink with
some Cr can be heated to give stable padparadscha orange-pink color.
- Very dark blue can be lightened by a few minutes at 1200oC
in an open, nonreducing vessel (Fe2+ oxidized to Fe3+).
- Whitish or pale blue containing masses of fine rutile
("silk"), called Geuda by Sri Lankans, can be heated in air to 1200oC
to remove blue; heating to around 1900oC will restore blue and remove silk.
- Irradiation can convert some pinks to padparadscha. Heating of
irradiated stones will restore the original color (yellow color center destroyed).
- A "surface
diffusion" method of treatment can be used to darken the color of pale blue sapphire, or turn white sapphire blue. Method
involves coating stone with TiO2 and Fe-oxide powder and heating to near
melting temperatures. Ti and Fe diffuse into outer few microns of stone, yielding a very
thin, skin-like layer of blue color. Immersion in oil will
reveal this (see strong color
zoning, darker color ring around the girdle and along facet junctions), as will bleeding
of color around cracks and pits.
- A more recent,
controversial variant, involving "bulk
diffusion", relies upon the diffusion of beryllium and/or other
light elements at very high temperatures into natural colorless or
pink sapphires, yielding yellow, yellow-orange and orange sapphire,
and padparadscha. Purplish red rubies treated by the same
technique may become a purer red.
- There are means to detect most heat treatments that include
examination of inclusions, dichroism, and fluorescence. Simple
techniques for detecting diffusion treatments rely on detection of
color
zoning of unnatural appearance, or of microscopic
recrystallized overgrowths (also termed "re-deposition").
Unfortunately, bulk diffusion that involves beryllium can
yield stones with no discernable color zoning or overgrowths,
requiring much more
sophisticated laboratory testing.
Pricing and
Valuation
- As per ruby. Darker blue stones that most jewelers carry
are not
as valuable as lighter-toned, bright, intense blue material. Darker stones have
become the commercial standard; most consumers would not recognize a Kashmir or Burma blue
as the extremely fine gems that they are.
- An excellent discussion of evaluating quality in sapphire can be
found in Chapter 10 of Hughes (1997). A table of famous
sapphires can be found there also.
- Finest quality 1-2 carat stones were in the following
wholesale ranges in 1990:
- Blue - $1,000-5,000/carat
- Padparadscha - $1000-2000/carat
- "Hot" Pink - $500-800/carat
- Orange - $300-600/carat
- Golden - $200-400/carat
- Purple - $200-500/carat
- Yellow - $50-100/carat
- Green - $50-80/carat
- More recent sources
of wholesale prices for the finest blue and pink sapphire are similar, with perhaps a
slight increases for pink sapphire.
- Prices for 1 to 2
carat, commercial grade blues and pinks are about 1/10th or less of the above prices.
- Blue stones are best with a violet overtone; a green overtone is
less desirable, as are gray-tinged or lightly colored blues.
- Large (15-35 ct) Kashmir
sapphire have recently (2000) sold at auction for $19,500 to $24,300
per carat.
Pinks should be of uniform color. Best is "hot" pink;
little to no lavender or brown overtone.
Purple should resemble fine amethyst, but somewhat redder.
Stay away from yellows with a brownish cast. Golden yellows or
orangish yellows are worth
more than pure yellows, but the former may be produced in large amounts with heat
treatment and the latter by diffusion/heating, so this generalization may no longer apply.
Greens should not show an olive or gray tinge, and should be free
of mottling or strong color zoning.
Synthetic blue sapphire and "padparadscha" are made by Chatham and several others. Sold in grades,
as per ruby. Chatham Gem Grade, 1989 wholesale to 1.5 carats costs $215/carat; at 10
carats $540/carat. Grade A stones sell for 1/2-1/3 as much in comparable sizes. These
synthetics are manufactured by a flux growth technique; sapphire made by flame fusion
(Verneuil process) are considerably less (pennies/carat).
Star Sapphire
Star sapphire is the result of reflection of
light from fine, oriented, rutile needles. Like star ruby, star sapphires may be heat
treated to high temperatures to dissolve the rutile and produce blue sapphires of good
clarity. Such has been the recent fate of much of the Sri Lankan Geuda material that might
have otherwise been uncutable (too dark and/or cloudy). Unfortunately, some reports state
that much of the good star material is now suffering the same fate. The same
characteristics apply to good star sapphire as to star ruby (see above). The best body
color is an intense
pure blue, but such natural stones are rare and one more frequently encounters blues
that are more gray. Fine blues are more translucent than even good gray-blues and are
cabbed thicker as a result. Such stones may appear to be "overweight" with bulky
bases, an important consideration when paying by weight but a necessity to maintain a deep
blue color. This is not the case for average blue stones, however, and you should not pay
for excess weight unnecessarily.
Synthetic star sapphire (and ruby) was first made by Union
Carbide ("Linde Stars") in the late 1940's. These synthetics, and those
currently being made by Nakazumi Earth Crystals in Japan, are superior in the straightness
and narrowness of the star and the outstanding deep blue body color. These characteristics
are sufficient to distinguish these synthetics from all but a handful of naturals; one
author states that the stars look so vivid and intense that they appear to be painted on
the stone rather than emanating from within it. These do not have to be cut as deep to
maintain color, and this can serve as a tip off as well.
Prices
- Finest quality blue star sapphire in the 5 carat range was selling
for about $2500-3000/ct (wholesale) in 1990.
- Commercial grade stones (whitish or grayish blue, poorer star) of
similar weight sold for $50-300/ct.
- Black star sapphire of even the finest quality generally sell for
under $50/ct (wholesale), ranging down to $1-2/ct for commercial quality stones of small
size.
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