Sienna is a form of limonite clay most famous in the production of oil paint pigments. Its yellow-brown colour comes from ferric oxides contained within. As a natural pigment, it (along with its chemical cousins ochre and umber) was one of the first pigments to be used by humans, and is found in many cave paintings.
Sienna, in and of itself, is sometimes referred to as "raw sienna", in order to differentiate it from "burnt sienna", which is a more common pigment than the raw form. The difference is in the process applied to burnt sienna, which is raw sienna heated to remove the water from the clay and redden its brownish colour.
The name derives from the most notable Renaissance location for the earth, Siena, Italy, and is short for terra di Siena, "earth of Siena". The mines used to produce this sienna petered out in the 1940s. Much of today's sienna production is still carried out in the Italian islands of Sardinia and Sicily, while other major deposits are found in the Appalachian Mountains, where it often goes hand-in-hand with the region's iron deposits.