A deep blue, usually opaque mineral, which makes up part of the lazurite in the gem lapis lazuli but is also used by itself as a gemstone. It was first discovered in Greenland (where it was incorrectly thought to be a type of feldspar) and did not become popular until after large deposits were discovered in Ontario and a large stone was given to Princess Margaret of England by her husband. (Hence the alternate name princess marble.) It's usually made into beads or cabochons or polished and used as an inlay.