Archon (hardstone)

Archon was a valuable hardstone found in Faerûn. It was also called Blue John.

Description
Archon was a low-grade fluorspar that was blue to purple and banded with white. Large specimens were used for carving because it was soft and easy to work. A typical price was 2 gp per lb (0.45 kg).

Powers
Archon gave off a faint greenish glow if a magically invisible creature or object came within 20 ft (6.1 m). This hardstone was only sensitive to magical invisibility, not creatures on the Ethereal plane or merely in disguise.