Maw-sit-sit is quite an unusual gemstone due to its curious name and variable chemical composition. It is a recent find, first identified in 1963 by the late Swiss gemologist Dr. Edward Gubelin. It was named after the village in Northwestern Burma that is close to the site where it was first found. Typically maw-sit-sit is green with distinctive dark-green to black veining.
Maw-sit-sit is considered to be a rock rather than a mineral, since it is composed of a number of different minerals, including kosmochlor (a mineral related to jadeite) and varying amounts of jadeite and albite feldspar. It is often classified as a member of the jade family but it is not really a variety of jadeite. Sometimes maw-sit-sit is called a "cousin" of jade, which seems appropriate.
The source location for maw-sit-sit lies in the foothills of the Himalayas, near the historic imperial jadeite mines of Northern Burma. To date, maw-sit-sit has only been found in this single location, making it a rare material indeed.
The dominant mineral species in maw-sit-sit is kosmochlor; sodium chromium pyroxene. About 60% of maw-sit-sit is made up of kosmochlor, which is the primary component of both the brilliant emerald green and the dark green-black patches. The other major component is chromium-enriched jadeite, which accounts for about 15% of the total material. Kosmochlor, once known as ureyite, also has an unusual name. It means "green from outer space," since the mineral was found originally in meteorites.
Maw-sit-sit has a refractive index ranging from 1.52 to 1.74, depending on its exact chemical composition. The lower end of the range is most common. It has a hardness rating of 6 to 7 on the Mohs scale and a density or specific gravity ranging from 2.5 to 3.5. Maw-sit-sit is translucent to opaque and is almost always cut as cabochons or carved.