Assignment 21 - Tanzanite, Iolite, Chrysoberyl, and Andalusite Flashcards
What are the choices a cutter has to make when fashioning tanzanite?
Due to tanzanite’s strong pleochroism, cutters must balance weight retention against quality of face-up color.
Why is heat treatment important to tanzanite?
Untreated, tanzanite is typically brownish.
When tanzanite is heated to approximately 600°C to 650°C (1100°F to 1200°F), its brownish color turns to a uniform blue. This is due to a change in the valence state of tanzanite’s vanadium component.
Since the resulting color is consistent from many viewing angles, it isn’t surprising that tanzanite is routinely heated to eliminate its brown, yellow, or green pleochroic colors.
How did political changes affect the tanzanite market?
Tanzania is the only commercial source of gem-quality tanzanite.
Because tanzanite comes from a single source, changes in supply drastically affect availability and prices.
As with many other gems, political instability disrupted the tanzanite market. In 1971, the Tanzanian government took over the operation of the mines. Supply dwindled to a trickle as the government strictly regulated the amount of tanzanite allowed into the marketplace.
In the late 1980s the Tanzanian government lost control of the mining area, and thousands of independent miners crowded the area.
By the early 1990s—partly because of renewed government regulation— supply became more regular and prices stabilized.
What is iolite’s most dominant feature?
Iolite has two distinctive features—a beautiful violetish blue through violet hue derived from iron, and striking, eye-visible pleochroism.
What qualities make iolite an alternative to tanzanite?
Iolite can be an affordable alternative to tanzanite because in comparable quality, it sells for about one-tenth the price.
What is alexandrite’s defining characteristic?
Alexandrite is the rare color-change variety of chrysoberyl.
Traces of chromium cause alexandrite’s color change.
What are the most-prized alexandrite colors?
Fine alexandrite is green to bluish green in daylight or fluorescent light and red to purplish red in incandescent light, with medium to medium-dark tone and moderately strong saturation.
What is the finest chatoyant gem?
Excellent hardness and durability combined with the sharpest cat’s-eye make chrysoberyl the most-prized chatoyant gem.
What are the qualities of cat’s-eyes at the three market levels?
The least expensive gems have incorrectly oriented, poorly focused eyes. They also have surface imperfections and prominent inclusions that disrupt clarity. Gems might also be almost opaque, or so transparent that it’s hard to see any chatoyancy at all. Bodycolor might be an unattractive brown or gray, and the quality of cut might be poor. Many stones are irregular, unsymmetrical cabochons with too much weight below the girdle.
Mid-priced gems usually display correctly oriented cat’s-eyes and good bodycolor with some evidence of milk and honey. Gems should have much better clarity and be free of surface imperfections. Some cutting variations are possible, but most gems should have reasonable proportions overall.
The best cat’s-eyes must have sharp, well-focused, correctly oriented eyes and fine bodycolor with clearly evident milk-and-honey effect.
What is unusual about andalusite’s pleochroism?
Andalusite’s pleochroism is so pronounced that its hues are often visible simultaneously through the gem’s crown, creating a mosaic effect.
What is chiastolite?
Also known as cross stone, chiastolite contains carbon inclusions that form the imprint of a dark cross, usually against an opaque white, gray, greenish, reddish, or light brown background. The impurities that create the cross also give chiastolite a slightly lower hardness than transparent andalusite.
In iolite, the most valuable main face-up color is
A. blue.
B. gray.
C. violet.
D. yellow.
A. blue.
Iolites with a predominantly blue face- up color are most valuable, so cutters orient the finished stones to display the desirable color.
Alexandrite was discovered in
A. 1830.
B.1897.
C.1955.
D. 1967.
A. 1830.
Alexander II was czar of Russia in 1830 when a previously unknown color-change gem was discovered in the Ural Mountains. It was named alexandrite, after the revered leader.
Where was tanzanite discovered?
A. Tunduru
B. Mangari
C. Merelani
D. Umba River Valley
C. Merelani
The most widely accepted account of tanzanite’s discovery is that, in 1967, a Masai tribesman stumbled upon a cluster of highly transparent, intense blue crystals weathering out of the earth in Merelani, an area of northern Tanzania in eastern Africa.
Which of the following is a major source of cat’s-eye chrysoberyl?
A.Vietnam
B. Colombia
C. Sri Lanka
D. Cambodia
C. Sri Lanka
