People in every era and from all walks of life have adorned themselves with the dramatic, radiant grace of colored stone jewelry. The world of colored stones is vast and varied, and a true understanding of them requires knowledge of both science and history. Here are just a few of the most popular colored stones today.
Alexandrite
Alexandrite is one variety of the
mineral chrysoberyl, named after
Czar Alexander II of Russia. It was discovered in 1831, in the Ural Mountains of Russia, and is composed of BeAl
2O
4.
Alexandrite is unusual because it is a grass green in daylight and raspberry red in artificial, incandescent lighting. The trace element
chromium is responsible for both the red and green of alexandrite. This same trace element is responsible for the green of emerald and red of ruby when is
substitutes for aluminum in the crystal structure. The coloring agents are dependent upon the wavelength of light and the crystal structure/chemical bonding; the
chromium in alexandrite is such that the color changes with wavelengths of light, from natural sunlight or fluorescent lighting, where is appears green, to indoor incandescent light, where is appears red.
Emerald
Emerald is the deep green variety of beryl, an
aluminum berylium silicate, Al
2Be
3(Si
6O
18). It is a cyclosilicate and often found in hexagonal primatic crystal forms. Beryl is 7.5-8 on the hardness scale and a vitreous luster. It has a conchoidal fracture and a brittle tenacity, which makes it sensitive to pressure and heat. Specific gravity is average to medium high, 2.66-2.87, and the refractive indices are 1.562-1.602. Although beryl may have an irregular distribution of color, the color is stable in light and heat. Asterism and cat's eye stones are possible. The best known variety of
beryl is emerald, a deep green color. The green coloring agent is the impurity element, chromium, and possibly some vanadium. Some believe chromium defines emerald, whereas beryl colored with vanadium is merely green beryl. The finest emerald are transparent but more commonly they are clouded with inclusions. Dispersion is 0.014 and emerald has distinct pleochroim, showing blue-green and yellowish-green. It has no fluorescence but
emerald shows bright red through the color or Chelsea filter.
Colombia is one of the largest commercial producers of emerald. Fine Colombian emeralds are highly regarded for their excellent color.
Zambia is also a
commercial source of emeralds with good clarity. Other sources include Afghanistan, Brazil, Pakistan, Russia, and Zimbabwe.
Opal
Opal is a hydrated silica, SiO2(H2O), that is lacking in crystal structure or is amorphous. The hardness is 5.5-6.5 and opal has a low specific gravity at 1.98-2.20. It has a conchoidal fracture and is brittle.
Opal contains water, from 3-30%, and this over time will be lost, diminishing the opalescence (Schumann, 1997, p. 150). There are basically three types of opal: precious opal (containing flashes of fire), the
orangish-red "fire opal" (named for its color, not flashes of fire), and common opal.
High heat or sudden temperature change can cause fracturing.
Opals are generally stable to light, but heat from intense light can cause fracturing (known as "crazing").
Opals are attacked by hydrofluoric acid and caustic alkalis. Loss of moisture, and crazing, can result from storage in airtight containers, such as safe deposit boxes.
Pearl and Cultured Pearl
Pearls have been valued for their natural beauty, no faceting or polishing required, for some 6000 years. Approximately 70% are strung and worn as necklaces.
Pearl quality is determined according to shape, color, size, and luster. Spherical are the most valued, but many pearls are irregular shapes, which are termed baroque or barrel pearls. Extreme dryness or moisture are both damaging conditions for pearls, as well as acids, perspiration, cosmetics, and hair spray.
The low hardness means pearl is easily scratched and therefore storage is an important consideration.
High heat can burn cultured pearls, or cause discoloration, splitting or cracking.
Pearls are generally stable to light but heat from intense light can cause dehydration and nacre cracking.
Pearls are attacked by many chemicals and all acids. Hair spray, perfume, cosmetics, and even perspiration can damage nacre.
Peridot
Peridot (pronounced pear-a-doe) is the gem variety of the mineral olivine, and also called chrysolite. It is a magnesium iron silicate, (Mg, Fe)2SiO4, with a hardness of 6.5-7, and specific gravity of 3.28-3.48. Cleavage is indistinct and the fracture, conchoidal. Crystal form is rare, although it is in the orthorhombic crystal system. It is doubly refractive with refractive indices of 1.63 and 1.695 ).
Pleochroism is weak and peridot has no fluorescence.
Peridot stability is Rapid or uneven heat can cause fracturing in peridot. The gem is stable to light, but is attacked easily by sulfuric acid, and less easily by hydrochloric acid.
Long-term exposure of perspiration may also attack peridot. Ruby
Ruby occurs in metamorphic rocks, dolomitic marble and gneiss, whereas sapphire may be found in sedimentary limestones, metamorphic marble, and igneous basalt, pegmatites, or andesite dikes. Corundum is very rarely mined from primary deposits, but typically mined from secondary alluvial or placer deposits, where this resistant mineral becomes the sand and gravel of stream beds.
Ruby and sapphire have been synthesized since the end of the 19th century for industrial and gem quality uses.
Synthetic star ruby and
sapphire have been marketed since 1947.
Heat can cause a change in color or clarity, it can also damage or destroy fracture- and cavity-fillings. Rubies are generally stable to light, but bright lights can cause oil to leak or dry out. Chemicals can harm fillings and remove oil; soldering flux containing boron and firecoat made with boric acid powder, will etch the surface of even untreated stones.
Spinel
Spinel is a good candidate for the title of “History’s Most Under-Appreciated Gem.” Some ancient mines that supplied gems for royal courts from Rome to China produced spinels, but they were usually confused with better-known stones like
ruby and sapphire.
Spinel’s color range includes violet, blue, orange, red, pink, and purple. Blue spinels are often grayish and subdued, but the best are a deep rich color. The reds can rival fine ruby. And the vivid orange to orange-red stones merit their trade name—flame spinel. Some spinels show color-change, usually turning from grayish blue in daylight or fluorescent light to purple under incandescent light.
Tanzanite
Tanzanite is a member of the zoisite group, as is thulite, the national gemstone of Norway. Thulite is an opaque, pink gemstone usually cut en cabochon, and is named after Thule, an old name for Norway. The
gem quality transparent zoisite, tanzanite, varies from a deep "sapphire" blue to an "amethyst" purple. It is now one of the birthstones accepted for December birthdays and is a calcium aluminum silicate, Ca2Al3(O/OH/SiO4/Si2O7).
Tanzanite is special-care gem for two reasons: sensitivity to thermal shock and the potential for cleavage. Sometimes the temperature change between the hot lights of the display case and the chilly glass countertop in an air-conditioned showroom can be enough to develop cleavages in
tanzanite.
Topaz
Topaz is a nesosilicate, Al2[(F,OH)2/SiO4], and 8 on the hardness scale. It is in the orthorhombic crystal system, which is reflected in the commonly found prismatic crystal form with orthorhombic pyramid terminations. It has perfect basal cleavage, as well as a conchoidal and uneven fracture.
Topaz is transparent and translucent, with a vitreous luster, and high specific gravity, 3.49-3.57. It is doubly refractive, 1.609-1.643, with a strong to definite pleochroism (with the exception of blue topaz, which as weak to none), and weak fluorescence. When
topaz is gently heated or rubbed, it becomes electrically charged.
Topaz occurs in cavities within granite or rhyolite igneous rock, pegmatite dikes, high temperature quartz veins, and/or alluvial deposits.
The largest faceted gemstone ever recorded is the "
Champagne Topaz." The gem was mined in Brazil, weighs 36,854 cts. and measures 34.9 cm x 15.5 cm x 12.1 cm.
Turquoise
Turquoise is hydrous copper aluminum phosphate, CuAl6[(OH)2/PO4]4 4H2O (Schumann, 1997, p. 170). It varies in color from a sky blue to blue-green or apple-green, and is frequently fashioned with the matrix rock and interspersed vein rock. The veined material is called
"spiderweb" turquoise. The blue color changes to green with high heat (of soldering) and exposure to light, perspiration, oils, cosmetics, household detergents (always remove turquoise rings before washing hands or dishes). It has a hardness of 5-6, cleavage, and a conchoidal fracture. The specific gravity is 2.31-2.84 and it is in the t
riclinic crystal system, crystals are rare. It has a waxy luster and both the fluorescence and pleochroism is weak.
Historically, the finest material was mined in Iran and is known as
Persian turquoise. This source is no longer commercially important. Today, the United States is the major source of turquoise. Other sources are China, Chile, Australia, and Mexico.
