A doublet is a type of

assembled gem An assembled gem (also called a composite gem) is a gemstone made up of other smaller gems. An assembled gem can often be a fake gem with a desirable piece of gemstone attached to pieces of inexpensive imitation gemstones. For example, a combinati ...

composed of two horizontal sections. It is sometimes used to imitate other, more expensive gems. Opal doublet

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

and

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

doublet uses a top portion of natural garnet fused to any color of glass to imitate a gem. The color of glass used in the doublet is all that is seen, as the garnet provides no color. If seen in reflected light, a separation line may be seen. A harder garnet makes the stones more durable. In the case of

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

doublets, a backing layer of

onyx Onyx is a typically black-and-white banded variety of agate, a silicate mineral. The bands can also be monochromatic with alternating light and dark bands. ''Sardonyx'' is a variety with red to brown bands alternated with black or white bands. ...

or matrix (

ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be c ...

) gives the more fragile opal layer support and can make the opal look darker and higher quality. Garnet and glass doublets were first used around 1850 when it was noted that molten glass would adhere to garnet. It was a popular imitation for all types of gems in many colors because the color of the glass became the only color that could be seen. They were still being produced into the early 1900s until actual synthetic gems were introduced.

References

Gemology {{gemology-stub