The name Tanzanite (after the East African state of Tanzania) was introduced by the New York jewelers Tiffany & Co. It is accepted by the trade, although scientists do not encourage a multitude of names for gemstones and refer to it as blue zoisite. In good quality the color is ultramarine to sapphire blue; in artificial light, more amethyst violet. When heated to 752-932 F/400-500 C, the yellow and brown tints vanish, and the blue deepens. Tanzanite cat’s eyes are also found. Some deposits in Tanzania near Arusha occur in veins or filling of fissures of gniesses. Tanzanite is a very valuable gemstone and is steadily increasing in value.Mohs’ Hardness: 6.5 – 7 Specific gravity: 3.35 Cleavage: Perfect Refractive Index: 1.691 – 1.700 Dispersion: 0.030
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The green variety of grossular garnet, discovered a few decades ago and found mainly in Kenya, near Tsavo National Park, is also known as Tsavorite.Appearance It is a light, verdant, or dark green similar to the color of the better green tourmalines and sometimes it is said, even comparable to African emerald. It has good luster. These gems are generally small and are given a round or pear-shaped mixed cut. These gems rarely exceed one carat and are never more than a few carats. Distinctive Features Tsavorite is distinguished from green tourmaline bt being singly refractive. Occurance It is very rare; being found mainly in Kenya and Tanzania, but also Pakistan.
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There is an intermediate group of garnets in the pyropealamandine series. It is a deep pink or pinkish-red color and is called rhodolite, from the Greek rhodon, “rose,” and lithos, “stone.”Appearance The pinkish-red color is its main characteristic. The gems have a good transparency and are almost always faceted , generally receiving a mixed, roughly oval or round cut. As always with transparent garnets, the luster is strong. Distinctive Features The color, luster, and single refraction typical of garnets paint quite a clear picture. Rhodolite is distinguished from corundum of a similar color by its lack of pleochroism and the fact that it does not floresce in bright light. Occurance Rhodolite is found in Zimbabwe, Tanzania, Sri Lanka, and the United States. It is not common.
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One of the rarest and most coveted of gemstones, Alexandrite is prized for its changefrom shades of green in daylight to shades of red in incandescent light. After its discovery in the Ural mountains in Russia, this gemstone became a favorite in Russia’s rulers, especially Czar Alexander II for whom it is named.Appearance Green: it can be almost emerald-colored, but it is more often yellowish or brownish green. Its main characteristic is the ability to change color if exposed to a light source rich in red rays; by candlelight or tungsten light, it turns red or reddish. Distinctive Features The changing color is an unmistakable characteristic. Occurance Found in limited quantities in the East African countries of Zimbabwe and Tanzania and also in Brazil and Sri Lanka. Value Fine stones are extremely valuable and are on a par with emeralds, rubies, and sapphires.
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Silicate of calcium and aluminum, belonging to the garnet group. The name grossular is due to the fact that some of these crystals resemble gooseberries, the latin name for which is grossularia.Appearance Grossular also has the typical crystal form of garnets. They vary from transparent to semi-opaque. The typical color is light (gooseberry) yellowish- green; but they can be strong to bluish-green, honey yellow to pinkish yellow, or even perfectly colorless. Like other garnets they have no cleavage. Occurance Grossular is a very rare mineral. The types used as gems mainly come from the gem gravels of Kenya; and Tanzania (green variety), Madagascar, Mexico, Canada, and the United States.
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| The name is of ancient origin. The Latin smaragdus appears, in fact, to have referred to the stone we call emerald, which is now considered as a distinct species.Appearance The typical color is beautiful distinctive hue known as emerald green and is due to traces of chromium in the crystal structure.
Distinctive Features The color is virtually unmistakable. It is only equalled by some very rare specimens of jadeite jade, which, however, is less transparent and has different physical properties. Occurance Increasing quantities of emeralds have been found in a series of small deposits in East Africa – principally in Zimbabwe, Zambia, and Tanzania. These are quite a strong color, sometimes with a blueish-green tint. Emeralds with similar characteristics also come from the mountains of India, Pakistan, as well as Urals. Value Stones of fine color, weighting more than 2 carats are among the most highly valued gemstones. Their price may equal or exceed that of diamonds.
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“The derivation of the name “spinel” is uncertain. It may mean “spark” (Greek) or “point” (Latin). Spinel occurs in all colors, the favorite being a ruby-like red. The pigments are chrome and iron. Large stones are rare and star spinels very rare.The blue variety is sensitive to high temperatures. THe dark green to black opaque spinel is called Ceylonite, also pleonaste (Greek – surfeit, because of its numerous crystal faces). The brown variety is called Piconite (French), yellow is known as Rubicelle (diminutive of French word for Ruby) and the pale red is known as balas ruby (after a region of Afghanistan). Spinel was recognized as an individual mineral only 150 years ago. Before then it was classed as ruby, because it also occurs with it. Some well-known “rubies” are really spinels, such as the 1.9in/5cm long, oval “Black Prince’s Ruby” and also the 361 carat “Timur ruby” in a diamond set necklace; both are in the English Crown Jewels. Both are uncut and only polished. The drop-shaped spinels in the Wittelsbacher’s crown of 1830 were also originally thought to be rubies. Main deposits in Burma (near Mogok) and in Sri Lanka (near Ratnapura). Less important deposits are in Anatolia, Afghanistan, Brazil, Thailand and the U.S. (New Jersey). The two largest spinels (formed as roundish octahedrons) weigh 520 carats each and are in the British Geological Museum in London.” Walter Schumann
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The name comes from the Latin rubrum, “red.” Like other red stones, it has been called carbunculus, or carbuncle meaning a small coal or ember.Appearance The color varies from fiery vermillion to violet red, but because rubies are pleochroic, different colors are also found in the same stone. The brightest red and thus the most valuable rubies (usually from Burma) often have areas full of inclusions in the form of minute rutile needles (or straws), which interfere with the light, producing a distinctive silky sheen known, in fact, as silk. When the silk is not heavy, the stones are clearer, more attractive, and even more valuable. Distinctive Features Rubies can often be distinguished by their immediate visible characteristics: a fairly obvious pleochroism, a distinct brightening of color in strong light, the silk effect (where present), and a considerable luster. While spinel can be a similar color and has a similar luster, it is not pleochroic, and turns much less bright in strong light and never displays a silk effect. Red garnet is not pleochroic and the color does not brighten in strong light; it has a similar luster, but when given a faceted cut often displays a dark, blackish area within the stone. Some caution is needed with garnets, which show wide variations in both density and refractive index. Garnet; however, is refractive, and examples with an index in the region of 1.76 have a lower density than that of ruby. Value The highest quality, best colored and most transparent stones (usually from Burma) weighting for example, 3-5 carats, can be valued as diamonds.
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| Gem and Rocks’ Guide to African precious and semi-precious stones provides both the connoisseur and the casual collector with a brief but thorough profile of rare varieties of African gems whose beauty and mystique have captured our imaginations. The gem profiles cover such aspects as appearance, distinctive features, occurance, and value. |