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. I

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.