Fluorspar

Fluorspar, also called fluorite, was a soft, easily workable stone found in many colors.

Description
These ornamental stones cleaved easily along four directions to form octahedrons. In the Realms, if a rough stone was pale blue, green, yellow, purple, pink or red, or was physically small, it was usually cut into faceted gems sometimes called "cabra stones". A purple-and-white banded variety was also mined in large blocks and was used for carving (see Hardstones: Archon also known as Blue John). A typical stone had a base value of 10 gp, with the higher valued stones tending to be the red and pink varieties.

It is also believed that the stone, (see Hardstones: Archon also known as Blue John). glows pale green when invisible creatures approach, and is therefore used for protection.