GEMS-1
(Last updated 12/23/07)
In February of 2004 I was accidently
(re)introduced to the world of colored gemstones. Since then my
curiosity about these shiny baubles has morphed into an unhealthy
passion for these marvelous minerals that now encompasses collecting
as well as learning everything I can about them. And although
I started using collecting as a way to discover what the gemstones
I knew only from pictures really looked like, the inventory of
duplicate stones I soon developed has led to another "hobby,"
the production of simple jewelry as a way for me to share my new
passion with family and friends, and perhaps increase their tolerance
of me until this latest obsession of mine runs its course (if
ever).
Altogether about 70 of the 4000+ known
minerals can yield colored gemstones, although only about 20 of
these are commonly encountered in jewelry. Pictured below are
examples of gemstones from my collection, as well as some of the
simple jewelry I've made. The emphasis in the descriptions of
the photos I've chosen (which don't really do any of the gems
justice) is on the chemical and physical properties of the minerals
from which these gemstones are derived, and in particular what
produces their color.
Diamonds, probably the best known of
all gemstones, are a crystalline form of the element carbon first
discovered in a river bed in India in about 500 BC, found again
in the jungles of Brazil in 1725, and then in other alluvial ("secondary")
deposits in Borneo and South Africa (1866). Up until the late
19th century, diamonds were genuinely rare, and world production
of gem quality stones amounted to only a few pounds per year.
But in 1870, huge diamond "pipes"
were discovered near the Orange River in South Africa. These were
primary deposits, "chimneys" of kimberlite rock that
marked craters where volcanic activity had brought diamonds to
the surface from where they had been formed up to 300 miles below.
Suddenly modern mining methods could be used, and diamonds were
NOT rare.
In fact, modern production of gem
quality diamonds now exceeds 30 million carats (>13,200
lbs) per year, and there are almost a billion carats in the hands
of the public. Why, then, is there the perception that diamonds
are rare and valuable? The answer is the De Beers cartel/monopoly
and its advertising campaign, which has not only created new markets
around the world for its product, but has convinced the public
that "diamonds are forever" and should never be sold.
[This story is chronicled in a marvelous book by Edward Jay Epstein,
"The Diamond Invention"; see http://edwardjayepstein.com/diamond/prologue.htm]
Usually thought of as colorless (another
legacy of De Beers), diamonds are, in fact, found in colors like
pinks, oranges, yellows, greens, blues, browns (with gimmicky
marketing names like cognac and champagne), even grays and blacks.
But unlike colorless diamonds, "fancy" colored diamonds
are truly rare, with perhaps one found for every 10,000 colorless
diamonds, and almost all under one carat. Such diamonds are thus
truly valuable - medium pinks sell for $25,000-$40,000/ct for
stones of ca a half carat, and a 0.90 ct vivid green recently
sold for $600,000 - even though they are unknown to most of the
public (eg, the Hope is probably the most famous diamond in the
world, but few people realize that it's a blue with a color so
deep that it's nearly opaque).
The Argyle diamond mine in Australia
(the world's largest), which started production in 1985, has changed
this situation somewhat because Argyle produces more colored diamonds
than all previous sources combined; and almost from its beginning,
Argyle decided to market its own diamonds rather than join the
De Beers cartel! As a result, colored diamonds came onto the market
at reasonable prices and in significant numbers, with a corresponding
increase in the public's appreciation of - and demand for - them.
Pictured below are a pair of 5mm round
diamond cut black diamonds, ca 0.4ct each, set as 14kt yg studs.
Natural black diamonds (but not black carbonados, which are aggregates
of tiny crystalline diamonds formed over half a billion years
ago by meteor impacts on the earth's surface) are colored by tiny,
plate-like inclusions of graphite (confirmation that all diamonds
near the earth's surface are slowly converting to this more stable
form of carbon now that they are under normal temperatures and
pressures). However, the blacks pictured below were produced by
high intensity neutron bombardment of faceted, low quality diamonds,
a new technique recently developed to meet growing market demand.
Rubies and sapphires are among the
most ancient, historically important, and best known of the colored
gemstones. Both, in fact, are the same mineral - corundum, or
aluminum oxide, Al2O3, which in pure crystalline form is colorless.
However, when ca 1% chromium (Cr) is introduced into the structure
of a corundum crystal, replacing some of the Al atoms, the result
is a red ruby.
Most commercial rubies come from one
of five geographical areas in the world - Burma, Madagascar, Tanzania/the
Serengeti, Thailand, or Vietnam. Because of slight variations
in chromium concentration and the presence of other trace metals,
the reds observed in rubies from different locations range from
a purplish or bluish red to an orange or yellowish red, often
with brownish undertones. The most prized color is displayed by
Burmese rubies - an almost spectrally pure, highly fluorescent
(due to the absence of iron) red with a faint undertone of blue
(often decribed as "pigeon's blood" red) that remains
constant in color and shows inner fire even in low light.
A pair of round cut Burmese rubies,
4mm in diameter, each weighing ca 0.3 carat (ct; one ct = 200mg;
141.75 ct = one ounce), set in 14 karat yellow gold (14kt =>
14 of 24 parts, or 58.3%, pure gold mixed with 10 parts of an
alloy, usually copper and silver) studs ...
...and a pair of Madagascar rubies,
6mm rounds, ca 1.2ct each, set in 14kt yg leverback style earrings
Most people think of sapphires as blue
but, in fact, corundum comes in almost every color besides ruby
red, and any corundum gemstone that isn't a ruby is a sapphire.
Most commonly sapphires come in hues of blue, green, gold, yellow,
orange, purple, and pink; and colors other than pink (which is
due to lower but still significant concentrations of chromium)
are produced primarily by charge transfer absorption effects between
trace amounts (usually less than 0.01%) of iron (Fe), titanium
(Ti), vanadium, and gallium that become dominant in the absence
of Cr impurities.
In blue sapphire, it is charge transfer
absorption effects between neighboring pairs of Fe2+ and Ti4+
in the corundum lattice that generate the blue color. The most
famous blue sapphires come from a deposit in the mountains of
Kashmir (discovered in 1880, long since mined out) and from Burma
(Myanmar); such stones are close to a pure spectral blue. Fine
sapphires are also found in Sri Lanka (Ceylon), Thailand, Madagascar,
Cambodia, Australia, Montana (the Yogos; sapphires from other
parts of the state are generally "fancy," ie, non-blue,
stones), and more recently, Tanzania and Brazil.
Pictured below are a pair of 0.6ct
round diamond cut pink sapphire studs in 14kt yg (left), and a
2.2ct, 9mm round Ceylon blue sapphire (right)...
...a pair of 0.5ct pear shaped golden
sapphires (left), and a pair of 1.2ct 6mm rd cabochon cut blue
star sapphires (right), set in sterling silver (s/s; 92.5% pure
silver alloyed with 7.5% copper, a standard supposedly set by
King Edward I of England in 1300, unchanged since then).
A glimpse of the fine needles of rutile
(TiO2) included within an 8+ct star ruby from Sri Lanka - it's
the refraction of light by such needles that causes the star effect,
or "asterism," in this stone, and also in the star sapphires
above
The mineral beryl - beryllium aluminum
silicate, Be3Al2(SiO3)6 - is the principal source of beryllium
in the earth's crust and, as such, is economically very important
(beryllium is used in areas as diverse as spark plugs, aircraft
alloys, and the nuclear industry). Colorless in pure form (called
goshenite; used to make spectacles in ancient times), it is also
the source of some of the best known and most important colored
gemstones when various impurities are introduced into its complex
zeolite-like crystal lattice: green emeralds when chromium or/and
vanadium are present; blue to greenish-blue aquamarine ("sea
blue") in the presence of ferrous iron, Fe2+, and golden
beryl with ferric iron, Fe3+ (and intermediate colors when mixtures
of both iron oxidation states are present); greenish-yellow heliodor
with Fe3+ and uranium; violet-pink to peach morganite with manganese,
Mn2+; and raspberry-colored red beryl (bixbite; among the rarest
of gemstones in the world) when higher concentrations of Mn2+
(as well as Fe and Cr) are present.
A pair of 0.8ct 7x5mm oval Columbian
emeralds (colored by chromium) in 14kt yellow gold (left), and
an 0.4ct 6x4mm emerald cut Zambian emerald (right; colored by
vanadium), examples of the best known member of the beryl family
A pair of 0.5ct 5mm round aquamarines
set in s/s leverbacks (left) and an unheated (as indicated by
the greenish undertones) 4.1ct 12x10mm modified Portuguese cut
oval aquamarine (right)
Among the lesser known beryls, a 2.3ct
10x8mm oval golden beryl (left) and an 0.75ct 7x5mm oval morganite
(right)
The garnet family of minerals is extremely
complex, consisting of at least 16 different silicates of the
general formula A3B2(SiO4)3 - where A represents the divalent
metals calcium, iron, magnesium, or manganese and B represents
trivalent metals such as aluminum, chromium, iron, or manganese
(and in the rarer garnets vanadium, titanium, zirconium, or silicon)
- that share the same crystal structure. Six of these minerals
(the pyralspite group, where A=Fe, Mg, or Mn and B=Al, and the
ugrandite group, where A=Ca and B=Fe, Al or Cr), together with
several binary and a few tertiary combinations, provide most gemstone
quality garnets. Although commonly thought of as a red stone (the
carbuncle of antiquity), natural garnets supposedly come in every
color except blue (and examples of even these have recently been
found), thanks to absorption produced by a wide variety of impurities
such as iron, magnesium, chromium, vanadium, and titanium that
can be incorporated into the garnet lattice. And although it is
a relatively common mineral, some garnets, eg, tsavorite and especially
demantoid, are among the rarest gemstones in the world.
A pair of 0.5ct 5mm round cut red garnets
(left; probably almandine, with A=Fe and B=Al) in s/s studs, with
a matching 2ct 8mm round stone set as a pendant, also in s/s;
and a pair of 1.1ct 8x6mm oval raspberry colored rhodolites (right;
a 2:1 mixture of pyrope, where A=Mg and B=Al, and almandine) in
s/s
Tsavorite is a grossular garnet, with
A=Ca and B=Al; the color is due to the presence of traces of vanadium
in this otherwise colorless gemstone. A very rare gem, and one
of my favorites, the total output of tsavorite is less than 1000
carats per year, with almost all stones under 1 carat. An intensely
colored (deeply saturated) 0.5ct 5x5mm trillion cut tsavorite
(left) and an 0.75ct 6mm round diamond cut (center). The 3.5ct
elongated cushion cut hessonite (right) is another grossular garnet;
the color is caused by iron as an impurity
A pair of 1.0ct 6x6mm trillion (triangular
shaped) mandarin colored spessartine garnets (A=Mn, B=Al) from
Namibia (left), and a pair of 1.6ct 6mm square princess cut canary
YAGs (right), set in 14kt yg studs. The man-made garnet YAG (yttrium
aluminum garnet), with A=Y, B=Al, was initially produced as a
colorless crystal in the late '50s; a neodymium-doped version
was used in laser research in the 1960s
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