||Intarsia is not a mineral of gem, but rather the way to create a stone from pieces of many stones. The name also applies to the wood process where a pattern or image is created from a number of different woods. In stone intarsia, the gemstone is created by gluing together a variety of colored stones together to form a pattern or in some cases an image.
The simplest intarsia are just alternating bands of color. In most cases the intarsia is backed by a solid stone to add strength to the composite. In that way they are also similar to opal doublets, where precious opal is glued to a backing to improve its thickness and strength.
Today most intarisa is created on a faceting machine which provides excellent precision and control of angles. Often a center stone is outlined with thin stripes of one or more secondary stones and then framed in a final material.
Many geometric shapes have been used in intarsia including stars, triangles, multi-gons, etc. Rarely, but not totally excluded, you will find circles in intarsia. My variety of intarsia is done freehand and freeform with domed tops more like cabochons. There are no hard and fast rules.