Sinhalite

Sinhalite was a rare find in streambed gravel, both active streams and ancient, dried up riverbeds. When polished cabochon or cut, these fancy stones were called sinhalas.

Description
In natural form, these crystals were rounded pebbles, tumbled and smoothed by water action. They were yellow-brown to pale straw-yellow and yielded gemstones up to 1 in (2.5 cm) in diameter. A typical specimen had a base value of 100 gp.

Powers
A sinhala could prevent magical darkness in a 20 ft (6.1 m) radius around the stone.