My previous article initiated a series on polymorphic aluminum silicate gemstones, beginning with andalusite, following with kyanite (disthene) and ending with sillimanite (fibrolite). Andalusite, kyanite, and sillimanite all contain identical amounts of aluminum, silicon, and oxygen, combined with other trace elements. I find it intriguing to note the differences in how the chemical formulas for these three minerals are expressed by many well-known authorities on gemstones.
That list includes: Joel Arem, Jaroslav Bauer, Max Bauer, Basil Booth, Vladimir Bouska, J.D. Dana, Malcolm Heuser, Cornelius S. Hurlbut, Jr., Cornelis Klein, J. Kourimsky, Michael O’Donoghue, Orlando Paddock, Walter Schumann, and Charles A. Sorrell. I once considered the possibility that those expressions might have been clues to indicate the form of the compound. However, I have not detected any clear pattern as yet. I located a very interesting discussion found on page 343 in the 19th edition of The Manuel of Mineralogy authored by Cornelius S. Hurlbut, Jr. and Cornelis Klein. In this reference, the chemical formulas for all three gems are expressed as A12SiO5.
Kyanite is usually of contact metamorphic origin, associated with its dimorph, andalusite, garnet, and corundum. Kyanite is also found in eclogites in kimberlite pipes. Flattened and elongated prisms of kyanite have been recovered from pegmatites, schists, and gneiss deposits all over the globe. The schists of St. Gothard in the Tyrol Mountains of Switzerland have been for many years the source for very fine kyanite crystals found in association with staurolite. The Machakos district in Kenya yields a very unusual colorless kyanite and some large green-banded blue kyanite crystals. Other kyanite sources include France, Italy, and India. Kyanite occurs in association with diamonds at Diamantina and also in the alluvial sands of Brazil. Some excellent dark blue facet grade kyanite has been found in a white quartz vein located near the summit of California’s Yellow Mountain. Deep blue and green gem quality kyanite crystals, measuring up to two inches in length, occur in Yancy County, North Carolina. Other states with deposits of kyanite include Connecticut, Vermont, Massachusetts, Virginia, Georgia, and Montana.
Kyanite abounds with uncommon attributes. Twinning is very common, as is parallel intergrowth with staurolite. The most characteristic feature of kyanite is the remarkable difference in hardness of the crystal faces. Depending upon the crystallographic direction, the hardness of kyanite varies from 4 to 5 and from 6.5 to 7. The mineral name “disthene”, from the Greek “di” for double and “sthenos” for strength, refers to this extreme variation in hardness. The prism faces of kyanite exhibit an unusual phenomenon also. Two of the narrow faces are striated vertically, and horizontal striations appear on the broad cleavage face. A plane of perfect parting oblique to the length of the crystal causes tiny horizontal cracks visible on the face. Kyanite is very brittle. A low cohesion makes it susceptible to splitting.
The combination of these characteristics create unique problems to faceters who work with kyanite. Most kyanite is translucent at best. Large lamellar cleavage masses of excellent color and with perfect cleavage surfaces frequently occur in matrix with other well developed crystals. Top quality kyanite gemstones make lovely, but very fragile gemstones. Faceted kyanite and kyanite cut en cabachon are more suited for pendants. I have also seen large blue crystals of kyanite wire-wrapped for pendants.
Kyanite sometimes exhibits magnetic properties similar to tourmaline. In “A Guide in Color to Precious and Semi-Precious Stones” authored by Jaroslav Bauer and Vladimir Bouska, the text reports the case where blue crystals containing admixtures of iron and chromium follow the lines of force of the Earth’s magnetic field like a compass needle when suspended on a hair.
Kyanite and andalusite is abundant in non-gem quality material. Kyanite has been used extensively in refractories and in the manufacture of highly refractory porcelains, ceramics, and spark plugs because of its resistance to high temperatures. Although kyanite is rarely free of inclusions, a few kyanite catseyes have been found.
The Greek word “kyanos” for blue provides the root for kyanite, however, individual color zoned crystals also occur in other colors such as pink. Dr. J. Kourimsky uses the name “disthene” to designate the blue variety of kyanite and “rhaeticite” to refer to the white and gray kyanite variety. At one time, French jewelers used the name “sappare” extensively when referring to kyanite, as a result of an error by a mineralogist from Geneva. The mineralogist, H.B. Saussure, Jr., misread a label attached to a specimen thought to be sapphire. Despite recognition of the mistake, the name “sappare” became widespread. That name is occasionally heard to this day when referring to kyanite
Kyanite Gemstone Properties
|Varieties:||transparent (by color)|
|translucent to opaque|
|Colors:||blue, blue-green colorless, white, gray, yellow, and pink|
|Phenomena:||chatoyancy (extremely rare)|
|Habit:||flattened bladed crystals, rarely found terminated|
|Fracture:||uneven, fibrous, and splintery|
|Specific Gravity||3.5 to 3.67|
|Hardness||directional 4.5 to 7|
|Refractive Index||1.71 to 1.734|
|Optic Character||biaxial negative|
|Pleochroism||strong for blue, colorless, and violet blue|
|Ultraviolet Fluorescence||variable; sw-inert; lw-weak red|
|Color Filter||no information|
|Solubility||no reaction to acids|
|Thermal Traits||infusible; avoid thermal shock|
|Inclusions||fibers of ilmenite, rutile, and hematite flakes|