Onyx

Onyx was part of the agate family of ornamental stones but was considered a semi-precious stone when found in pure black, pure white, or in straight bands of both colors.

Description
This semi-precious stone was carved for jewelry and decorative housewares and was hard enough to wear well. It was often used for sculpting figurines (including the magic onyx dog). A typical specimen had a base value of 50 gp.

Powers
There were "nine secrets" that could be enchanted to become an ioun stone and onyx was one of them. Physical contact with onyx helped reduce complications and pain during childbirth.

Rumors and Legends
Unenchanted onyx was believed to cause bad luck when worn or carried. Elminster has confirmed this suspicion.