GEMS-3
(Last updated 1/4/09)
Olivine is the name given to the
entire series of mixed magnesium iron silicates of the formula
(Mg,Fe)2SiO4 between the two end member minerals forsterite (all
Mg) and fayalite (all Fe). Olivines (peridotites) contain the
same chemical makeup as the molten magma in the Earth's mantle,
and are thus the earth's most common rock by volume. Hawaii's
famous green sand beaches are made up of olivine, indicating their
volcanic origins. Olivine has also been found in many iron-nickel
meteorites, often as significantly sized crystals, sometimes making
up over 50% of the meteorite's volume; and olivine was also recently
found by the Rover on Mars.
Olivine's gemstone variety, peridot,
displays a distinctive greenish-yellow to yellow-green color;
the best quality peridot usually has an iron content of ca 15%
and includes nickel and chromium as trace elements. One of the
most ancient of gems, many of Cleopatra's favorite "emeralds"
were found to be peridot when subjected to modern analysis, and
probably originated from a deposit on the Red Sea's volcanic island
of Topazos (St. John), which has been mined for almost 4000 years
(the island also gave us the name for the mineral topaz). Today
most peridot comes from the San Carlos Apache Reservation in Arizona,
as well as from deposits in China, Pakistan, Burma, and Korea.
A pair of 1ct 7x5mm oval Arizona
peridots set in 14kt yellow gold studs (left), and a beautifully
cut 8ct peridot from Pakistan (right)
Tourmaline, commonly used to indicate
a family of gemstones, is actually the name of a group of isostructural
complex aluminum borosilicate minerals with the general formula
AX3Y6(BO3)3Si6O18(O,OH,F)4, where A is sodium or calcium, X can
be aluminum, iron, lithium or magnesium, and Y is usually aluminum,
but can also be chromium or iron. One mineral in the group, elbaite
- Na(Li,Al)3Al6(BO3)3Si6O18(OH)4, provides the majority of gem
quality stones, but the opaque black mineral schorl (A=Na, X=Fe,
Y=Al), the most common tourmaline, is also used in jewelry, and
translucent crystals of the related brown dravite (A=Na, X=Mg)
and green uvite (A=Na,Ca, X=Mg) are sometimes faceted as well.
With a complex structure that can
easily accomodate many different trace impurities, tourmaline
comes in more colors than any other gemstone. Its typical thin
elongated crystals are frequently color zoned, and can often have
over a dozen different colors (or shades of the same color) along
their length. Crystals can also be concentrically zoned, eg, a
popular variety known as "watermelon" tourmaline has
a pink center, a white ring around that, and green around the
outside of that. Available in colors from white (colorless) to
black and in shades from pastel to vivid (more than 100 separate
hues have been identifed), a recent trend in the marketplace is
to name tourmaline by a simple color designation, eg, yellow tourmaline.
However, particularly desirable colors have historical names that
are still in use. Vivid pink to red stones that retain their color
in incandescent light are called rubellite (left; rich in Li,
free of Mg and Fe), vivid blues indicolite (center), rich greens
verdelite (or elbaite), vivid greens (with a very high Cr content)
chrome tourmaline (right), and particularly vivid (described as
"electric" or "neon") greenish blues (containing
both copper and gold) "Paraiba", after their Brazilian
state of origin (found only in 1987, Paraiba tourmaline is already
one of the most sought after and expensive gemstones in the world).
Among the least popular tourmalines
for jewelry are the rare colorless gems (called achroite) and
the abundant yellows (left), although a particularly vivid canary
yellow (containing traces of magnesium) discovered in 2000 in
Malawi may change the popularity of the latter. In contrast, pinks
(containing manganese, and with color centers from natural irradiation)
such as this 6.0ct 14x10mm oval pink tourmaline (right) from the
Pala mine north of San Diego, are among the most popular colors.
Tourmaline also produces various
types of phenomenon stones, including cat's eyes and bi- and tri-coloreds
such as this 1.2ct elongated emerald ("tourmaline")
cut "watermelon" tourmaline (left; with green, colorless/white,
and red segments); it is also often found as inclusions, as in
this 10ct 16x12mm emerald cut tourmalinated quartz (right) containing
needles of black schorl encased in rock crystal (right)
The mineral zirconium silicate -
ZrSiO4 - is found worldwide, but Thailand, Cambodia, Sri Lanka,
and Burma produce the majority of zircons (gem material) and the
greatest variety of colors - usually brown, but also colorless,
yellow (jargoon), orange, red (jacinth), green, and blue. Zircons
always contain hafnium (1-4%), usually uranium (ca 300ppm - its
slow decay to lead provides a way of using zircons to date rocks
billions of years old) and thorium (ca 150 ppm) as well. The "self-irradiation"
caused by the decay of these radioactive impurities results in
damage to the gemstone's atomic lattice, producing red-brown to
amber color centers and eventually altering the ordered crystal
to an amorphous glass. Zircons thus affected are termed "metamict".
Almost all zircons are heat-treated to remove the color centers
produced by such radiation damage. Colorless zircons, which have
little uranium, are NOT the same material as manmade cubic zirconia
(CZ; a form of ZrO2), but are often used as a diamond simulant
because they resemble diamonds in both luster and fire.
A 2.75ct 9x7mm oval honey zircon
(left), a 2.1ct 8x6mm oval blue zircon (center), and a 3.3ct 10x8mm
oval green zircon (right) from Sri Lanka (green is generally associated
with metamict gems)
Spinel is the name given to a group
of ca 20 oxide minerals that have the general formula AB2O4 (where
A represents a divalent metal ion such as magnesium, iron, nickel,
manganese, or zinc, and B represents trivalent metal ions such
as aluminum, iron, chromium, or manganese) and very similar crystal
structures (that have the same high symmetry as diamond).
The gemstone quality mineral of
the spinel group, magnesium aluminum oxide - MgAl2O4, is also
called spinel. Pure spinel is colorless, but chromium impurities
can give it a red color that mimics ruby; in fact, many famous
spinels (eg, the 170ct Black Prince's Ruby that forms the centerpiece
of the royal crown of England and the 352ct Timur Ruby, also owned
by Queen Elizabeth) were thought to be rubies until recent times.
Though less common than red, spinel comes in other colors as well:
pink, yellow to orange (rubicelle), violet (almandine), blue (cobalt
spinel), green (chlorospinel, containing traces of iron and copper,
and zinc-rich gahnite), brown (picotite, or chrome spinel), and
black (pleonast). The best spinels still come from Burma (the
historic source of spinels), but fine gemstones are also found
in Sri Lanka and Tanzania.
A pair of 0.5ct 7x5mm oval red spinels (left), an 0.4ct pear shaped
blue spinel (center), and a 1.75ct 8.5x6.5mm oval violet spinel
(right)
Spodumene, or lithium aluminum silicate
- LiAl(SiO3)2, is a colorless mineral that contains three fairly
recently discovered gem varieties: triphane - colorless to yellow,
kunzite - pink to violet, and hiddenite - an unusual green color
that is unlike either peridot or emerald. Triphane is fairly common
but seldom seen as a gemstone (except in collections), while true
hiddenite, found only in Alexander County, North Carolina, and
only in small sizes, is so rare that it, too, is seldom seen outside
of collections.
On the other hand, a commercially
significant deposit of kunzite was discovered in 1902 in the Pala
region of California (in conjunction with the pink beryl morganite
and pink tourmaline), and large deposits have since been found
in many other parts of the world; today most kunzite is mined
in Brazil, Afghanistan, and Madagascar. Although considered a
real challenge to the gem cutter because of its multiple cleavage
planes, tendency to splinter, and strong pleochroism - making
proper orientation critical during faceting, kunzite's lovely
lavender color - from the presence of manganese (and enhanced
by natural or artificial irradiation to produce color centers)
- has made it an attractive and desirable gemstone. However, because
its color centers are metastable, kunzite should be protected
from heat and from exposure to strong light, either of which can
cause its color to fade.
A 7.6ct 14x13mm pudgy pear cut kunzite
from Afghanistan
Zoisite, calcium aluminum silicate
hydroxide - Ca2Al3(SiO4)3(OH), is a structurally complex green
stone that looks similar to aventurine. Known for nearly two centuries,
a pinkish variety called thulite is used for beads and cabochons,
and a bright green chromium-containing variety from Tanzania that
also contains red corundum (ruby) is used as an ornamental stone.
However, in 1967 a transparent, highly pleochroic, deep blue gemstone
was found in the Merelani foothills of Mt Kilimanjaro. Much to
the surprise of both minerologists and gemologists, this new gemstone
turned out to be a zoisite; named tanzanite (and initially marketed)
by Tiffany's, tanzanite has since become one of the most popular
(and expensive) colored gemstones in the world.
A 28x25mm oval ruby-in-zoisite cabochon
(left), and a 1.15ct, 6.5mm round tanzanite (right)
Cordierite, magnesium aluminum silicate
- Mg2Al4Si5O18, is normally gray, but can occur as a leek-green
gemstone called praseolite, and turns a deep blue-violet as its
ferrous ion content approaches 10%. Cordierite also produce a
highly pleochroic, blue gemstone called iolite (or water sapphire
or dichroite), or can display a reddish color when it contains
inclusions of hematite. Gem grade iolite is found primarily in
Sri Lanka, India, Madagascar and Burma.
Chrysoberyl, or beryllium aluminum
oxide - BeAl2O4, is not a well-known gemstone mineral even though
it can produce extremely brilliant and durable gems (its hardness
is just a bit less than that of sapphire) and one of its varieties
is among the most expensive gemstones in the world. Colorless
when pure, transparent chrysoberyl most often appears yellow,
yellowish green, or brown due to iron (Fe+3) impurities.
A 1.3ct 8x6mm oval chrysoberyl (left),
and an 0.5ct 6x4 oval cat's eye (right; "cat's eye"
alone always refers to a chrysoberyl; the names of other cat's
eye gems must include the type designation as well, eg, opal cat's
eye)
A 1.6ct 7x5mm oval alexandrite cat's
eye from India (left), and a pair of 2.4ct 8x8mm heart shaped
lab grown alexandrites (right)
Apatite, calcium chlorofluorohydroxyl
phosphate - Ca5(Cl,F,OH)(PO4)3 (the mineral of teeth and bones),
is actually three different minerals depending on the predominance
of chlorine, fluorine, or hydroxyl; but as all three minerals
are usually present in every specimen, they are usually considered
as one due to the difficulty in distinguishing among them. Apatite
is widely distributed in all rock types, usually as small disseminated
grains or cryptocrystalline fragments. However, large well-formed
crystals, typically green, but also yellow, blue, reddish-brown,
and purple, have been found in certain contact metamorphic rocks
and can be cut as gemstones. The softness of apatite once precluded
its widespread use in jewelry, but the discovery of a neon blue-green
variety in Brazil that mimics the color of Paraiba tourmaline
has sparked renewed interest in apatite as a gemstone. Gem quality
material is also found in Mexico, Canada, Germany, and Russia.
A pair of 0.80ct 7x5mm oval teal
colored apatites
The mineral lazurite, sodium calcium
aluminum silicate sulfur sulfate - (Na,Ca)8Al6Si6O24(S,SO4), can
form deep blue crystals, but is more commonly found in massive
form and combined with other minerals, usually pyrite and calcite,
in a rock (mixture of minerals) called lapis lazuli. First mined
6000 years ago, the finest quality lapis - a translucent blue
containing no calcite or pyrite that looks very much like Bakelite
plastic - still comes from a remote mountain valley in Afghanistan,
although lower quality lapis is also found in Chile, and to a
lesser extent Siberia, Italy, and the US (in California and Colorado).
Other common gemstone minerals still
not described are the cryptocrystalline jades - jadeite and nephrite
- and turquoise, the organic gemstones pearl and amber, and some
less common gemstone minerals such as andalusite, sphene, amblygonite,
and kyanite. I hope to correct this problem in the not too distant
future.
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