Fluorspar

Fluorspar, also called fluorite, was a soft, easily workable stone found in many colors. The pink and red varieties were sometimes called cabra stones.

Description
These ornamental stones cleaved easily to form faceted gemstones, but this treatment was reserved for small samples and specimens of the most popular colors: pale blue, green, yellow, purple, pink, and red. A purple-and-white banded variety known as archon or Blue John was mined in large blocks and used for carving. A typical stone had a base value of 10 gp, with the higher valued stones tended to be the red and pink varieties.

Powers
All varieties of fluorspar gave off a faint green glow when invisible creatures or objects were within 20 ft (6.1 m). Fluorspar did not react to ethereal objects or creatures, nor to things or beings merely disguised.