Peridot
(Pear-ih-doe) is the gemological name for the mineral Olivine ([Mg, Fe]2SiO4).
Gem-grade peridot is usually close to the Mg end-member (12-15% Fe2SiO4
present) of olivine; the Fe present is a chromophore that gives a dark or medium
yellowish-green color. Some of the best, brightest green peridot contains traces of Ni or
Cr, which are also chromophores. Peridot is a birthstone for August, along with sardonyx.
The name Chrysolite is an older German name that was once applied to all gem
olivine; the term is not as commonly used today but is sometimes used to denote a more
yellowish-green to greenish-yellow stone, as opposed to a dark yellow-green or brownish
green (olive green) common to peridot. Peridot is sometimes also referred to by jewelers
as "evening emerald".
The name peridot is from the French peritot,
meaning unclear, and is probably with allusion to the presence of numerous
inclusions. Though flawless stones of smaller size (up to about 5 carats) are common
(far more common than flawless emerald of similar size), larger clear stones are rare.
Properties
- Crystal System: Orthorhombic
- Habit: Crystals very rare; usually massive, nodular.
- Hardness: 6.5-7
- Toughness: fair to good
- Cleavage: 1, imperfect, parallel to c-axis.
- Fracture: conchoidal
- Specific Gravity: 3.3-3.4
- R.I.: 1.650-1.690
- Birefringence: moderate, 0.0036
- Dispersion: medium, 0.02
- Pleochroism: weak to none
- Color: yellowish-green to greenish yellow; tone
correlated with Fe content. Fe contents above 15% give a very dark or muddy color.
- U.V. Fluorescence: none
Distinguishing Properties
- From green tourmaline by lack of dichroism, R.I.
- From green zircon by R.I., lower birefringence, and S.G.
- From emerald by color (more yellow in peridot) and inclusions,
R.I.
- From green glass with polariscope.
Sources
Olivine is restricted in occurrence to
metamorphosed impure dolomites and to basic or ultrabasic (low in Si, high in Mg and Fe)
igneous rocks. Nearly all gem peridot is derived from the latter. Unlike most other gems,
olivine is highly susceptible to chemical weathering and thus does not survive very long
at the surface in wet climates. This fact probably accounts for the very limited number of
known gem localities, and their restriction to areas having arid or semi-arid climates.
Most gem peridot, in fact, has its source from three localities: Zagbargad Island (also
know as St. Johns Island) in the Red Sea; Myanmar (Burma); and Arizona.
- Zagbargad
Island (Egypt), Red
Sea - ancient source; mentioned by Pliny in 1500 BC
- In veins that cut ultramafic rock (peridotite, dunite)
- Mines originally worked by primitive methods; were improved prior
to W.W.I. $2 million mined in 4 year period preceding W.W.I. Mines nationalized in 1958.
- Little or no mining currently taking place.
- Peridot Mesa, San
Carlos Apache Reservation, Gila Co., Arizona
- until very recently, 80-95% of world peridot
production was from this locality
- From ultramafic nodules in basalt; small, granular crystals
- Exclusive mining rights held by various Apache families
- Inclusions include glass blebs, chromite in "lotus leaf"
habit
- Crystals typically small (1/4-1/2"), yielding gems of 2
carats or less. Gems over 5 carats rare.
- Myanmar (Burma), north of Mogok Stone Tract
- Bright green euhedral crystals in weathered serpentine (ultrabasic
rock). Renowned for color, considered the finest peridot in the world.
- Have a "sleepy" appearance due to numerous small
crystallite inclusions.
- Once an important source, not presently (see Ruby)
- Others; minor production
- New Mexico (Kilbourne Hole), Pakistan,
Mexico (Chihuahua), Norway, Ethiopia, Australia, China, South Africa.
Shaping and treatment
- Step or brilliant cuts. Lotus leaf inclusions in larger pieces may
be placed parallel to table to give an attractive, iridescent sheen.
- No info. on treatment, if any.
- Peridot is relatively soft and can be easily scratched if set in a
ring. Some material is also quite sensitive to rapid temperature changes, an important
consideration when mounting or repairing peridot jewelry.
Pricing and valuation
- Included or flawed stones not worth much. Best stones are
flawless, with good yellow-green color. Stones of this character in 1-10 carat sizes run
about $20-$100/carat; in 15 carats or larger $70-$250/carat. Brownish green colors and
included stones are $1-10 carat in all sizes. Although a beautiful gem material in its own
right, peridot prices suffer from comparison with other green gems, emerald in particular,
which commonly have a more desirable bluish tint with less yellow.
- Both Burmese and Egyptian peridot are regarded by most
connoisseurs as superior in color to Arizona peridot. This, as well as limited production
from these areas, translates to higher prices. Larger gems (10 carats or more) show a
better depth of color than do smaller ones. Fine
Pakistani peridot has also sold at a premium, fetching as much as
$300/ct for gemstones larger than 20 carats.
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