Shadowgem

Shadowgem is a mineral.

Source
Miners can extract Shadowgems from Copper and Tin veins, and Jewelcrafters can extract Shadowgems from Copper and Tin ore. Other potential sources include mob drops and chests.

Shadowgem as an ingredient
Shadowgems are used in the following recipes:

Blacksmithing

 * Big Black Mace
 * Deadly Bronze Poniard
 * Heavy Bronze Mace
 * Runed Copper Breastplate

Engineering

 * Shadow Goggles
 * Goblin Jumper Cables

Jewelcrafting

 * Bronze Band of Force
 * Gloom Band
 * Ring of Twilight Shadows
 * Thick Bronze Necklace

Leatherworking

 * Shadowskin Gloves