The Stygian Gemstone is one of a set of 4 gemstones used to obtain either a Coffer of Whispers or Armbrace of Truth.
A Stygian Chest spawns every time The Dreadspawn Maw is defeated. The chest contains 1-8 Stygian Gemstones for each party member, depending on how many Lords of Anguish have been defeated so far and the game mode you are in.
Location: Gate of Anguish
Collecting: 1 Margonite Gemstone, 1 Stygian Gemstone, 1 Titan Gemstone, 1 Torment Gemstone
Location: City of Torc'qua, Ravenheart Gloom, Stygian Veil, The Ebony Citadel of Mallyx, The Foundry of Failed Creations
Collecting: 15 Margonite Gemstones, 15 Stygian Gemstones, 15 Titan Gemstones, 15 Torment Gemstones