The transparent gems are known for their striking luster and brilliance along with their color and hardness. Most transparent gems are oxides of aluminum, beryllium, or magnesium, with a few containing silica. Some transparent gems such as ruby and sapphire are identical in mineral composition and only differ in color. The value of these stones is determined largely by their transparency, lack of flaws, brilliancy of color, and size.The corundum gems, including ruby and sapphire, are all rare forms of alumina (Al2O3) that vary widely in color. Green, purple, and yellow corundum are known as oriental emerald, oriental amethyst, and oriental topaz. In addition, synthetic rubies, sapphires, emeralds, and similar gems of large size and fine quality are made by fusing fine alumina with appropriate mineral pigments under high temperatures.
Ruby is the most valued of all gemstones, and the deep red varieties are prized even more than diamonds. It is a vivid red gem of the mineral corundum, which is second in hardness to diamond on the Mohs hardness scale.The color, attributed to the presence of a small amount of chromium, varies from deep red to pale rose red, and in some stones it shows a tinge of violet. The oriental ruby, mined mostly in Burma, has very limited distribution, which accounts for its extraordinary high value. The ruby was considered by the Hindus the king of the precious stones. It was also thought to bestow good fortune when worn on the left side of the body.
Sapphire is the serene blue variety of corundum and is essentially the same mineral as ruby, differing only in color and being slightly harder.
It also occurs in vivid green, violet, and yellow hues.The color of the ever-popular rich blue sapphire is attributed to the presence of oxides of chromium, iron, or titanium.The Montana variety has a peculiar electric blue color. Star sapphires are unusual in that they reflect light in a figure of a six-pointed star.
Emerald, known for its deep green color, is a gem of the mineral beryl and should not be confused with the oriental emerald, which is actually an emerald-colored sapphire. Beryl is an important ore of beryllium as well as a major gemstone.The crystals are hexagonal and are principally found in granite pegmatites. When colored green, beryl is emerald, when colored blue or blue-green it is aquamarine, and when colored pink it is morganite. Emerald colors vary from light green to dark green and are due to the presence of chromium. Compared to the other transparent gems, the emerald is relatively soft, only slightly harder than quartz. Emeralds were mined in Egypt as early as 1650 B.C., and Cleopatras mines, located on the Red Sea coast east of Aswan, yielded precious emerald gems to adorn the queen of Egypt. Emeralds were once thought to have therapeutic value, curing such afflictions as poisoning, diseases of the eye, and the possession of demons.
Zircon is one of the most extraordinary gems. It is fairly common in igneous rocks, but rare as a gemstone. Zircon crystals found in granite are enormously resistant to erosion and tell of the earliest history of the Earth, when the crust first formed some 4.2 billion years ago. Zircon is also an important ore for zirconium, hafnium, and thorium. A rich golden brown variety is the most magnificent of all gemstones possessing this color. The natural colors range from clear varieties, which are favorite substitutes for diamonds, through shades of yellow and brown to deep brownish red. Brown zircon crystals can be altered to a rich blue variety by heating them in the absence of air, a potential that makes them more highly prized gems. Tourmaline, often called the rainbow gem, displays the widest range of exquisite colors of all the gems.
The crystals are often long and are unique in having curved triangular cross sections. Not only does tourmaline appear in almost every color of the rainbow, but a single crystal might be half one color and half another or show three different colors resembling a candy cane.The profusion of colors is due to a complex chemical composition, including perhaps a greater variety of chemical elements than any other mineral.The most valuable colors are a clear ruby red and a bright sapphire blue.Tourmaline is very common in pegmatites, where it sometimes occurs in crystals of enormous size. Because of its unique crystal structure, tourmaline has a strange electrical property. It becomes positively charged at one end and negatively charged at the other end of the crystal when heated. The static electricity attracts such objects as small bits of paper similarly to the way a comb drawn through the hair does.
Garnet is a rare gem of silicates of various metals with a typically red, brown, yellow, white, green, or black color and a glassy to resinous luster. Garnets are a common group of closely related silicate minerals, containing calcium, magnesium, iron, and aluminum, combined with silicon and oxygen. Crystals are usually 12- or 24-sided or any combination to yield 36 and 48 sides. Demantoid is a brilliant green variety that resembles an emerald in color. Pyrope, sometimes called precious garnet, is deep red and used in jewelry. Perfect specimens are found in South African blue earth in kimberlite pipes associated with diamonds and make exceedingly fine gems.
Staurolite is an iron-aluminum silicate often found with garnets in metamorphic rocks such as schists, phyllites, and gneisses and in pegmatites. An unusual inclusion of staurolite mineral in a South African diamond implies that it came from a piece of continental crust that was recycled through the mantle. Staurolite is brown to black and almost always occurs as stubby prismatic crystals usually an inch or less long, but in excellent specimens they are up to two inches long.Twinning is common, and when occurring at right angles the twin crystals form a perfect cross. These crystals, called fairy crosses, are broken or weathered from the bedrock and are sold as charms or souvenirs.Transparent crystals are rare and are cut as gemstones.
Peridot is a clear yellow-green gem of the mineral olivine, which is the most common member of a group of silicates found in many igneous rocks. It is an iron-magnesium silicate and the simplest of the dark minerals. Olivine is found in igneous rocks rich in magnesium and low in quartz such as basalt and gabbro. It has small, sugary grains, and crystals are relatively rare, although occasionally some up to several inches long have been found. Peridotite is the rock name for this mineral, which is significant because a very similar rock type is mined from volcanic structures called kimberlites for diamonds, indicating the mineral originated within the Earths mantle.
Topaz is commonly a yellow gemstone but ranges in color from pale yellow to brown. It has been very popular for jewelry since the 16th century.The rare pink topaz is much admired, especially the deeper hues. Topaz has a remarkably slick surface and a slippery feeling that distinguishes it from other minerals. Some large crystals, weighing upward of 600 pounds, have been found in pegmatites, with colors varying from yellow to blue, green, violet, and colorless.The yellow, yellow-brown, and blue-green varieties often occur in beautiful crystals that make valuable gems.The most important commercial source of the gemstone is Brazil.The mystical powers of topaz are believed to increase when the moon is new or full in the sign of Scorpio, at which time the wearer is able to receive strong impressions from astral sources.