Kyanite
Nearly all gemstones have a single hardness value, no matter where it is measured on the crystal. Kyanite is unique in having two hardness values. This characteristic is just one of many challenges that face a lapidary working with this gem.
Not only is kyanite beautiful, but its properties make it one of the most fascinating of gemstones. The colours you will see in gem-quality material are shades of blue, white, green, grey and occasionally black. The name kyanite is derived from the Greek word kyanos, meaning blue, and the best material has a clear, royal blue colour that looks like sapphire. Most other kyanite has a streaky colour distribution.
Gem-quality material is transparent to translucent, and the lustre ranges from vitreous to pearly. Chatoyant stones occur rarely.A great deal of bead and cabochon material contains inclusions of quartz, pyrite crystals, hematite flakes and fibres of ilmenite and rutile.
Faceting Kyanite
Kyanite has two hardness values: 4.5-5 and 7. The lower value runs along the crystal's length, parallel to the direction of the cleavage. The harder value runs across the crystal's width, perpendicular to the cleavage. Lapidaries working with this stone have to constantly adjust their pressure and speed so that they make progress grinding the harder areas yet avoid overcutting the softer areas. Kyanite has a moderate birefringence (double refraction) and is strongly trichroic, showing colourless, violet-blue and cobalt blue.
Pricing Kyanite
The availability of kyanite is good and there is a wide choice of quality and prices in bead form. The better quality is not particularly cheap, but the colour and clarity more than compensate for the price.
Working with kyanite
- Kyanite has an uneven, splintery to fibrous fracture and a perfect cleavage; it is full of small internal stress cracks that occur as the crystal forms at high pressure. Its low cohesion means that horizontal striations commonly run parallel down the broad face of a crystal, and the strong cleavage plane causes tiny cracks to form along the crystal's length. This makes kyanite a rather brittle gemstone that is susceptible to splitting. It requires careful handling, so any polishing should be done at low speed and ultrasonic and chemical cleaners should not be used. However, kyanite is not sensitive to heat.
- Cutting is limited by the nature of the material: sharp corners tend to fray, so angular or geometric shapes are not really suitable. Lower-grade kyanite beads need to be checked for any scuffs, splits and damaged drill holes or corners.
- Although kyanite poses some problems for lapidaries and jewelry makers, it should not be automatically rejected for use as a jewelry stone. There is some beautiful faceted kyanite that has been set into pendants and earrings. Natural blue gemstones are rare and gem-quality kyanite gives the colour of sapphire without the price. The higher-grade material, which is less flawed, is a safer jewelry stone.
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