Please enable JavaScript.
Coggle requires JavaScript to display documents.
Tanzanite, Iolite,
Chrysoberyl & Andalusite (TANZANITE
坦桑石 (Sources…
Tanzanite, Iolite,
Chrysoberyl & Andalusite
TANZANITE
坦桑石
Treatment
- Typically brownish
- Almost always heat-treated to bring out its more attractive blue and violet hues.
Routinely heated to eliminate its brown, yellow, or green pleochroic colors
Sources & Market
-
Because tanzanite comes from a single source, changes in supply drastically affect availability and prices.
2 distinct markets for tanzanite: One for large, fine gems, & One for small, calibrated stones.
-
TanzaniteOne came to dominate the tanzanite market
- bought AFGEM & Has sightholder system
- operating as a publicly traded company and a subsidiary of Richland Resources
When moderately priced commercial-quality blue sapphire is available, it’s more marketable than commercial-quality tanzanite.
-
-
-
Quality
-
Due to tanzanite’s strong pleochroism, cutters must balance weight retention against quality of face-up color.
- 6 to 7 on the Mohs hardness scale, fair to poor
- One direction of cleavage & sensitive to thermal shock
IOLITE
Sources & Market
- Sri Lanka and in several areas of Africa, including Kenya, central Tanzania, and the island of Madagascar, where a significant deposit was discovered in 1994
- usually found in alluvial deposits
Iolite can be an affordable alternative to tanzanite because of incomparable quality, it sells for about one-tenth the price.
A silicate of aluminum, iron & magnesium
Quality
- Violetish blue because of trace element: iron
- Striking eye-visible pleochroism
- Cutters try to fashion iolite so that grayish, pale yellow, or brownish yellow pleochroic colors don’t show face-up.
- Any face-up brownish or yellow can lower the value considerably
- Mohs Scale: 7 - 7.5, fair toughness
- District cleavage in on direction
Iolites with a predominantly blue face-up color are most valuable, so cutters orient the finished stones to display the desirable color.
CHRYSOBERYL
金綠寶石
ALEXANDRITE
-
-
“emerald by day, ruby by night.”
Quality
Fine alexandrite - 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.
Strongly Pleochroic, 3 colors are typically green, orange & purple-red
The alexandrite effect
- the color-change phenomenon
- Only the hue changes, tone and saturation remain the same
Mohs Scale - 8.5, Excellent toughness & no cleavage
Source & Market
Although Russian alexandrite mines produce little today, the rare and vivid hues of Russian alexandrite still set the standard for fine color.
A large deposit of alexandrite discovered in Brazil in the early 1980s briefly increased alexandrite’s availability.
Currently, fine alexandrite is so scarce that it has virtually disappeared from the gem marketplace except as an expensive collector’s gem.
-
-
Imitations
-
In 1987, Japan’s Kyocera Corporation was the first to produce a commercially available synthetic cat’s-eye color-change alexandrite
CAT'S-EYE CHRYSOBERYL
Quality
-
-
Fine-quality
- The chatoyant band is distinct, straight, and silver-white, well focused.
- True "milk and honey" Effect: One side of the gem is a semi-transparent to translucent slightly brownish yellow, and the other a yellowish green.
Excellent hardness and durability combined with the sharpest cat’s-eye make chrysoberyl the most-prized chatoyant gem.
Source & Market
Sri Lanka is the home of the finest cat’s-eye deposits.Other cat’s-eye sources are Brazil, Myanmar’s Mogok Stone Tract, Zimbabwe, and Madagascar.
Because it withstands rough wear and is subtly colored, cat’s-eye is traditionally marketed as a man’s gem.
Imitation: Cat's-eye quarz, yellow cat's-eye tourmaline
ANDALUSITE
紅柱石
Pleochroic Andalusite
- Andalusite’s pleochroic colors tend toward reds, greens, and oranges.
- Andalusite’s pleochroism is so pronounced that its hues are often visible simultaneously through the gem’s crown, creating a mosaic effect.
- Mohs scale: 7 - 7.5, fair toughness
Sources: Brazil, Sri Lanka & Spain
Other Varieties
Chiastolie - Inclusions cause the distinctive cross-shaped pattern on its surface, typically cut as cabochon or tablet
Sources: Siberia, South Australia, Myanmar, Zimbabwe, and the US states of Arizona and California.