Kefnium

From Memory Alpha, the free Star Trek reference

Kefnium is an element which comprises a large portion of the outer shells of the creature known as "Junior" and its kin. Kefnium may therefore have been a significant part of their diet in space. A great quantity of the element was also found in an asteroid belt near the Alpha Omicron system, where the creatures were known to frequent. (TNG: "Galaxy's Child").