Onyx is a common Gemstone, primarily of use for bartering, or sale to Traders.
It's said when Shar walks upon the earth, she leaves a path of onyx behind her.
Properties
Miscellaneous Items
Rarity: Common
Weight: 0.01 kg / 0.02 lb
Price: 35 gp 
UID
LOOT_GEM_Onyx_A
Where to find
Found throughout the game as set-dressing and random loot.
External Links
Onyx on the Forgotten Realms Wiki