GEM WORLD: Faceted Stones

Corundum has a chemical composition of aluminum oxide. This group, consisting of rubies and sapphires, are the hardest of all known colored stones outside of diamonds. The most valuable is ruby. Only the red color is reserved for ruby, all other colors belong to sapphire including pink and reddish pink. Rubies have also been called "carbuncle" which means a small coal or ember.

Chrysoberyls are usually made of beryllium aluminum oxide and have a Mohís hardness of 8-1/2. The most common color is golden yellow to greenish yellow. An extremely rare and valuable stone of this group is alexandrite. Alexandrite has the ability to change color usually from a greenish color in sunlight or florescent light to a reddish color under candlelight, incandescent, or tungsten light.

Spinels are made of magnesium aluminum oxide and have a Mohís hardness of 8. Many spinels have been mistaken for rubies. They have also been called carbuncles. The color of spinel can vary from red to pink, violet red to a pale lilac, violet blue, or a blue to black. Spinel stones comparable in color to rubies are quite valuable.

Topaz is a silicate of aluminum containing fluorine and hydroxyl and it also has a hardness of 8. Most people are familiar with blue topaz, but it also comes in colors like pink, violet pink, yellow, and colorless (sometimes called silver topaz). Blue topazís are normally produced by irradiation. Fairly intense pink stones and the "imperial" or "precious" golden yellow, peach, and sherry colors can be quite valuable.

Beryls are composed of aluminum beryllium silicate and have a hardness of 7-1/2 to 8. There are many varieties. Emerald is a well known variety that comes in the colors of emerald green, light green, yellow green, and dark green. Emeralds can be quite expensive. Only the finest qualities are transparent. Inclusions, healed cracks, and foreign crystals are not classified as faults, but are evidence as to the genuiness of the stone. Aquamarines are light blue to dark blue or blue-green. A deep blue is the most desirable color. Golden beryl is a lemon yellow to a golden yellow variety. Goshenite is colorless. Heliodor is a gold color. Morganites are pink, peach, and sometimes violet. Bixbites are strawberry red, quite rare, and expensive.

Garnets are a group consisting of many differently colored minerals with similar chemical composition mainly containing a silicate of magnesium, iron, and/or aluminum. The stones of this group are pyrope, almandine, spessartite, grossular, dematoid, and uvarovite. Some other popular names for garnets in this group are: rhodolite, hessonite, melanite, topazolite, andradite, and tsavorite. The hardness of garnets varies depending upon the variety. It may vary from 6-1/2 to 7-1/2.

Zircon is a silicate of zirconium sometimes containing radioactive thorium and/or uranium. Its hardness can range from 6-1/2 to 7-1/2. Brown is the most common color. It can be colorless or found in colors like red, green, blue, and yellow. Electric blue stones are the most valuable of this group.

Tourmalines are mainly composed of aluminum borate silicate and have a hardness of 7 to 7-1/2. There are many color varieties. Achroite which is colorless is rare. Rubellite varies from pink to red. The ruby red color is the most valuable. Dravite varies from yellow brown to dark brown. Verdelite comes in all shades of green and is the most common of all cut tourmalines. Emerald green is the most valuable. Indicolite comes in all shades of blue. Siberite varies from lilac to violet blue. Schorl which is black is very common and rarely used in jewelry. Tourmalines are well known for their unusual bi-color, tri-color, and watermelon varieties.

Spodumenes are a lithium aluminum silicate, having a hardness of 6 - 7, and comes in two varieties. Kunzite is the pink to violet color variety of this group. Hiddenite is the yellow-green to green-yellow color variety. The emerald green color is the most valuable of this group. These stones have perfect cleavage and can fade in sunlight.

Quartz is composed of silicon dioxide, has a hardness of 7, and comes in many varieties. Rock crystal is colorless. Smokey quartz is brown to black. Amethyst is violet and is sometimes referred to as purple quartz. Citrine can vary from a light yellow to a gold-brown color. Prasiolite is a leek-green variety. Rose quartz varies from a pale pink to a strong pink. There are many other varieties that are usually made into cabochons such as chrysoprase, heliotrope or bloodstone, tigerís eye, hawks eye, aventurine, prase, blue quartz, chalcedony, cornelian, sard, jaspers, and agates.

Tanzanite which is a blue zoisite is composed of calcium aluminum silicate and has a hardness of 6-1/2 to 7. The same stone can display three different colors each one from a different direction. This characteristic is called pleochroism. Some of the colors it may display are sapphire blue, amethyst, violet, violet blue, and/or brown. It was first discovered in 1967. Since then it has become very popular.

Peridot,sometimes called Olivine, is a magnesium iron silicate. The color varies from yellow green to green. The yellower type are usually called chrysolite. Peridot is not resistant to sulphuric acid and may become damaged upon contact.

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