Official State Mineral of Delaware
Delaware recognized sillimanite as the official state mineral in 1977. All State Minerals
Sillimanite is widespread in the metamorphic rocks of the Delaware Piedmont and large masses of sillimanite can be found as stream-rounded boulders at the Brandywine Springs State Park. These mineral boulders are remarkable for their size and purity. Delaware sillimanite has a fibrous wood-like texture and could potentially be cut into non-faceted gems showing a "cat's eye" effect.