A complex sulfur-containing sodium aluminum silicate.
The natural variety is a very costly blue pigment with exceptional chemical stability except in the presence of acids.
Ultramarine is famous for having been the most expensive pigment. It was more expensive than gold during the Renaissance. First used in 6th century Afghanistan, the pigment found its most extensive use in 14th and 15th century illuminated manuscripts and Italian panel paintings, often reserved for the cloaks of Christ and the Virgin.