Although sometimes used as a gemstone for carvings and jewelry, Sillimanite is actually a mineral and was so adopted by the Delaware Legislature in 1977.
Sillimanite is a polymorph with two other minerals; Kyanite and Andalusite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Sillimanite is the rarest of the three trimorphs. A variety of sillimanite called "Fibrolite" is what is predominately found in Delaware in its common fiberous massive form.
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 can be cut into non-faceted gems showing a "cat's eye" effect. A brown, gray, white, or pale green mineral, it is used as a gemstone and for ornamental carvings. Another historical name, Fibrolite, comes from its appearance; the official name Sillimanite was given in honor of Yale's first professor of mineralogy, Benjamin Silliman. Similar formation are found in Mexico, New Mexico and Arizona where Mexican Indians fashioned many tools from the Sillimanite masses.