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Delaware recognized sillimanite as the official state mineral in 1977. Sillimanite is widespread in the metamorphic rocks of the Delaware Piedmont and large masses of sillimanite can be found as boulders at Brandywine Springs. These mineral boulders are remarkable for their size and purity.
Sillimanite has no industrial value and is not mined as an ore or raw material. Delaware sillimanite has a fiberous wood-like texture and could potentially be cut into non-faceted gems showing a "cat's eye" effect.
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