The granular form of ilmenite, a titanium/iron oxide ore, produces a speckle in clay and glaze but is rarely used in glazes because it settles more readily than finer, less dense ingredients and, in a fired glaze, typically remains as hard, touch-detectable granules. It is suitable as a clay body speckling agent.
For more information on ilmenite, see the Digitalfire.com Reference Library of ceramic materials.