Black opal

Black Opal is a greenish type of opal with black mottling and gold flecks. Usually found in ancient hot springs, the gem is usually tumbled smooth and cut cabochon. The phrase in the North "Black as a black opal" means, effectively, not very black (or evil) at all, and is used to describe good-hearted rogues and similar individuals who would be embarrassed by praise. A typical specimen has a base value of 1000gp.

Uses
A black opal (but not a normal opal) could be used to enhance the effects of a wand, that contained a spell of the subschool of force. Only one could be built in per wand and once built in the wand's force spell magic either got harder for the target to resist or the strength behind the spell increased.

This was a very attractive method for wand crafters to increase their wand's effectiveness. Normally crafting stronger wands required more spellcasting skills from the crafter, which he may not possess. The other aspect was more interesting. Normally, crafting any kind of magic item burnt some of the inner essence of the crafter, the stronger the item the more was burned. Adding a gem was a way to increase the effectiveness of a wand without sacrificing one's own essence.

Effectively, adding a black opal was a way for the crafter to substitute his lack of skill and need to burn his essence to create a strong force-wand with money (1000 gold as mentioned above).

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