Black opal

Black opal was a dark version of opal found in ancient active or dormant hot springs.

Description
This stone had a dim greenish matrix with blotches of black and flashes of gold. It was usually smoothed by tumbling and cut cabochon. A typical specimen had a base value of 1,000 gp.

Powers
Black opals had an affinity for force effects and could be used in the creation of wands that were particularly good at delivering force spells. If powdered and mixed with crushed orl, it formed a powerful explosive that could be touched off by an open flame, if done properly. The blast was extremely devastating to anything within 10 ft (3 m) and could still do significant damage to creatures and objects up to 30 ft (9.1 m) away.

One recipe for a helm of darkness called for thirty-six black opals and a black sapphire.

Trivia
Calling a rogue (or anyone who operated in the shadows, away from the spotlight) "black as a black opal" was an ironic way of saying that they had a good heart without causing them too much embarrassment.

The drow deity Vhaeraun showed his pleasure or displeasure by making his followers stumble on or change the quality of a black opal in their possession.