COMPOSITES - GARNET

composites - garnet-topped doublets and soude emeralds

Imitations may also be composites - made of more than one part. For example, a piece of green glass topped with a slice of red garnet can be used to imitate an emerald or green garnet. The garnet-topped doublet (GTD) is made of two parts and the junction between them can usually be recognized by the difference in lustre. There may also be char­acteristic bubbles in the glass, which are not visible in the garnet.

When viewed through the table facet, the gemstone appears green, but when viewed from the side, or immersed in water, the red slice of garnet will be more obvious. GTD can be made to imitate gemstones of all colours by altering the colour of the glass beneath the gemstone top. Another composite is the soude emerald, made of two layers of colourless quartz, sandwiching a thin green layer of gelatine or glass.

opal doublets and opal triplets

opal often occurs in thin seams with the precious opal forming just a thin sliver or slice. An opal doublet (made of two parts) is made by cementing a piece of precious opal (showing a play of colour) to a 'backing' layer of 'potch' opal (non-precious opal). Opal doublets may also have a quartz, chalcedony, glass, or plastic base. Opal triplets have a protective cover­ing of quartz above the opal in addition to a backing layer.

opal simulants

The play of colour seen in precious opals is due to interference of light by its internal spherical structure. In 1974 the French scientist Pierre Gilson unveiled the first laboratory-produced opal. Gilson imitation opals can be distinguished from natural opals by their patchy appearance and the mosaic-like junctions between coloured 'grains'. The US scientist John Slocum created a glass opal, known as 'Slocum stone'. The patches of colour in Slocum stones have a slightly crumpled look when magnified.