Doublets are fine slices of opal backed with a base, to emphasise the gem’s natural beauty. Bursting with colour, doublets are created to resemble solid black opal. When creating our doublets and triplets, we only use gem-quality opals. Doublets and triplets are water-sensitive and should not be immersed.
Take a look at this: A doublet opal is thin layers of opal attached to a backing. This is usually a common stone or black opal. It's made from thin opal to create a solid product.
An opal doublet or triplet can be a great budget option for a dark opal appearance. These stones are an amazing price alternative to solid black opals, which are ten times the cost. Solid black opals are rare and valuable, so these cheaper alternative stones serve a practical purpose in making dark opals affordable for everyone.
When you examine an opal closely, if it looks like different colors are visible in layers, it's probably a doublet. A doublet is a thin layer of opal attached to a dark backing. If it looks like there are two layers of opal glued together, it's a triplet. It may also have a third layer on top, which is clear and domed.
Source. Geology.com
A gemstone's cut should be well-rounded. The dome should be dome-shaped. It gives the best play-of-color, and it makes the stone look great from every angle. If a stone is flat, it's more likely to break during setting into jewelry.
Gilson opal is a term used to describe imitation opal, which isn't only created in the lab but actually contains minerals that are not standard in natural opals (such as plastic in Slocum stone).