Black sapphire

Black Sapphire was a rare variety of sapphire that was a deep, rich black with yellow or white highlights. They were found mostly in the South with the greatest concentration in the Great Rift. These stones were highly valued by dwarves. A typical specimen had a base value of 5,000 gp

Uses
A black sapphire (but not a normal sapphire) could be used to enhance the effects of a wand, that contained a spell of the subschool of darkness. Only one could be built in per wand and once built in the wand's darkness 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 sapphire was a way for the crafter to substitute his lack of skill and need to burn his essence to create a strong darkness-wand with money (5000 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 sapphires in their possession.