lapidary

Lapidary manuals deal in alluring terms like ‘chatoyancy’ [the cat’s-eye effect] and ‘asterism’ [as seen in a star sapphire], both related to the play of colour in the dispersion of light – how it may split inside the gemstone and be affected by its pavilion angles. So beryl, corundum, and tourmaline crystals will splinter light into two rays, at right angles; the quality named ‘birefringence’. And ‘pleochroism’ means the differentiated colours in these split rays, as happens in the gemstones andalusite, axinite, corundum, iolite, spodumene, tourmaline, and tanzanite. The professional lapidary will cut each stone accordingly, to get its most productive faceting. Dark garnets, for instance, will need only a shallow cut for the best dispersion of their limited internal light. But the differing properties of gemstones mean that only certain types can be faceted. Whereas the ‘cabochon’ cut is a smoothing, domed style which suits amber, cornelian, and onyx, for instance.

Denise Riley, 'On the lapidary style'