For some time, this tough, durable gemstone has been either considered a collector’s stone or used industrially (in the manufacture of spark plugs!). It is finally becoming available to jewelry makers, who will, no doubt, appreciate the subtle colours and optical effects that sillimanite has to offer.
Treatments and imitations
In its massive, noncrystalline form, fibrolite can resemble jade; “gemmy” faceted sillimanite can resemble iolite or pale sapphire.
Aside from the sought-after faceted blue gemstones, sillimanite/fibrolite is relatively inexpensive. Faceted material is still quite rare, but cabochons and cat’s-eyes have become much more available.
Working with sillimanite
•Sillimanite is a relatively tough and durable gemstone; it is resistant to mechanical shock and chemicals and fairly tolerant of heat. However, one single good cleavage runs parallel to the length of the crystal, so if pressure is applied in the wrong place the stone can split in two.
•The fibrous material is slightly softer than the crystalline variety and has an uneven, splintery fracture. It is suitable for carvings and cabochons.
Types of sillimanite
Named after Professor Silliman of Yale University, sillimanite is very much a stone of two parts. When faceted, it is a lively, brilliant gem with a glassy to diamond lustre that comes in a range of subtle transparent colours: grey, blue, violet-blue, blue-green and brown-gray. The most sought-after colour is blue, which comes from the ruby-rich Mogok region in Myanmar (formerly Burma). Sillimanite is pleochroic, particularly so in the blue stones, which show yellowish pale green, dark green and blue from three angles.
The other face of sillimanite is very different and even has a different name — fibrolite. The name derives from the long slender sillimanite crystal prisms, which can occur in parallel groups that resemble fibres. This fibrous nature is more common in the massive opaque form of fibrolite, but traces of the perpendicular fibres can also often be seen in the translucent to transparent cat’s-eye material. Fibrolite possesses a silky lustre, which is common to chatoyant stones, and can display a razor-sharp white “eye”.