GARNET

Posted by Site Moderator Webmaster on Saturday, March 25, 2017

Duncan Miller

Garnets are among the easiest gem materials to facet. They have no distinct cleavage, although some crystals have a parting that causes them to fracture into thin slabs. The rough often is in globular shapes, which is good for weight recovery. When choosing rough, avoid being fooled by fake material. Red glass is sometimes covered in adhering deceptive ‘grit’ to mimic natural nodules. Illuminated from behind or the side with a torch, the characteristic internal swirls and round gas bubbles of glass give away the game. The only real issue lies in selecting red or brownish red rough that is not too dark to bother faceting. With dark garnet rough doing the ‘white paper’ test is necessary. You should be able to see good colour through the stone laid on white paper in strong, but indirect, light if you want to cut stones with a full pavilion. Larger, fewer pavilion facets are preferable on dark stones. But dark red garnets can be faceted into rose cuts, with no pavilion but a polished, flat base that reflects light coming from the sides.

Garnets occur in literally all the colours of the rainbow, including purplish blue garnets discovered recently in Madagascar. Bright red pyrope garnets, usually in small sizes, occur in the southern African kimberlite pipes, and have been marketed as so-called ‘African rubies’, a trade name that should be discouraged. Green demantoid and yellow andradite garnet come from deposits near the Erongo mountain in Namibia. Garnets in a wide variety of red, orange and yellow colours are found in the gem deposits of East Africa and Nigeria. These varied colours of garnets are due to the fact the garnets are a family of chemically distinct minerals, with varied compositions and diverse colouring, that also can form mixtures with each other. This causes some variation in the density and refractive indices of the different gem varieties, and some minor differences in optical and physical behaviour.

The brown and golden varieties of grossular garnet, known as hessonite or cinnamon stone, often have a swirled interior appearance that can look similar to glass. The Namibian demantoids sometimes have bands of slightly differing refractive index, looking a bit like cleavage planes, but they are not. They also are somewhat softer than other gem garnets and the variation in hardness in different directions can take you unawares, resulting in over-cut facets. The softer facets have a rougher surface before polishing, and these can need more polishing, throwing out the meets. You need to be aware of this and avoid it by cutting these facets slightly short. I polish Namibian demantoid garnets on a Gearloose Lapidary Greenway lap. This has chromium oxide embedded in the polymer surface, and avoids getting polishing agent stuck in the voids and cracks that these stones often have. Other than this, garnets present no problems in faceting and polishing. The other garnet gem varieties I cut on 600 mesh or 1200 mesh, prepolish larger stones on 3000 mesh on copper, and polish on a Batt lap, similar to tin/lead, with aluminium oxide (Linde A)

Some garnets, including some Namibian demantoids, show a subtle but distinct change of colour between fluorescent and incandescent light, making them difficult to photograph accurately.