The easiest way to sum up the chemical composition of the subsaturate group is to say these have extra oxygen ions. Examples are kyanite, staurolite, sillimanite, andalusite, and topaz. I just recently acquired a sample of the kyanite and found it attractive with its underlying blue shimmer. My specimen comes from North Carolina. Staurolite is found in a range from New England to Georgia, also in Sweden, and often forms cruciform crystals that can be made into cross jewelry. Sillimanite has the same composition as kyanite but a different structure and is a bit harder. Andalusite also has the same chemical composition as kyanite and sillimanite. It forms at lower temps and is sometimes faceted and used as a gem. Everyone knows topaz. It has a similar structure to other subsaturates but is otherwise unrelated. Topaz can be found in a variety of colors. The amber colored is the birthstone for November.