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 combinat ...

composed in two 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. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

and

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

doublet uses a top portion of natural garnet fused to any color of glass to imitate a

gem A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, a ...

. 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 silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

doublets, a backing layer of onyx or matrix ( ironstone) 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 {{decorative-art-stub