Black sapphire

Black sapphire was a rare variety of sapphire found mainly in south Faerûn, particularly around the Great Rift where the dwarves of the Deep Realm prized them greatly.

Description
These stones were an inky, lustrous black that reflected white or yellow highlights and were facet-cut in the same way as their blue cousins. A typical specimen had a base value of 5,000 gp.

Powers
When cut and polished, a jewel-quality black sapphire prevented time-related spells from operating within a range of 30 ft (9.1 m). Spells such as temporal stasis, time stop, and spells from the arcane chronomancy school or divine time sphere all ceased functioning, either temporarily or permanently, depending on the nature of the spell. These gems could also be used in the creation of exceptionally powerful wands that stored spells like darkness and related magics. The darkness effects produced were either more difficult for the target to resist or the strength of the spell increased, again, depending on the nature of the spell.

The shadow magic item helm of darkness used a large black sapphire and thirty-six black opals in its construction.

Trivia
The drow deity Vhaeraun sometimes showed his favor by allowing his followers to stumble upon a black sapphire, or increase the value of one already in their possession. He could also express his displeasure by lowering the quality of any black sapphires they carried.