Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course

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Kyanite

Kyanite is an unusual and rarely seen gem material with, in the best specimens, a gorgeous blue color. The name is from the Greek: kyanos = blue, but colors range through grey, green and white. A notable trait of the species, in regards to color, is that there is almost always noticeable zoning or uneveness. An aluminum silicate, it is polymorphic with two other species: Andalusite and sillimanite. (Polymorphic species share the same chemical formula, but each has its own crystal structure -- the most famous example of this situation is diamond and graphite.)

[A cabochon showing top color, and one with pronounced color zoning which the lapidary has chosen to display as an attractive band of color centered between colorless zones]

[A rare transparent piece, a colored zoned, chatoyant cabochon]

 

Another oddity of this material, and one that makes it extremely difficult to cut, is variable hardness. Parallel to the long crystal axis, the hardness is about 4.5, perpendicular to the same axis the hardness is about 6.5. Many gems show this feature to a slight degree (in fact if diamond didn't, it couldn't be cut by diamond abrasives), but in kyanite we see the most extreme case of this characteristic.

Widely, but sparsely, distributed in metamorphic rocks, and alluvial deposits dervived from them, sources include Brazil, the USA, Switzerland, Russia, India, Zimbabwe, Madagascar and Kenya. Very few specimens of a level of transparency and clarity useful for faceting are found, and due to the perfect cleavage, and hardness variability even fewer survive the cutting process intact. A small percentage of material shows some chatoyancy.

[A beautiful Brazilian kyanite crystal in matrix: Image courtesy of www.irocks.com]

Faceted stones or cabochons can be set and worn if given reasonable care. Kyanite is both heat and shock sensitive, so steam and ultrasonic cleaners should be avoided and settings should be protective. Nonetheless, with its beautiful color and interesting properties this gemstone will amply reward the adventurous collectors and jewelry lovers who own and/or wear it.

[A chatoyant kyanite cabochon protectively set in gold with a blue moonstone]


Value Considerations

No established value ranges exist for kyanite due to its rarity in the market, but, in general, the bluer the better. Fancy cutting and large size geometrically increase its value. Clarity (especially with respect to evenness of color) also raises value. In cabochons chatoyancy is highly desirable.


Gemological Properties:

Chemical Composition: an Aluminum Silicate: Al2SiO5

Crystal System: Triclinic

RI: 1.71 - 1.73

Density: 3.58

DR: .017

Dispersion: .20

Fracture: Splintery

Cleavage: perfect in one direction

Luster: vitreous to pearly

Hardness: Both 4.5 & 6.5


Stones Currently Available:

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Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course