It is always a dark place, so get ready for some evil-based tiles. The only way to leave is death, or by killing the Onethaci.
Deathrock: The surface, and the first few layers below the surface re made of this material. It is black, with the texture of stone, it needs at least a Molten Pickaxe to break.
Shadowsilt: The middle layers add this to the mix, even though it is still mostly Deathstone down on the middle layer. It is purple, with the texture of sand. Requires a Nightmare pickaxe at least.
Chaos Gemstone: One of the ores. It is used to create Chaos Anvils which are the only type that can create Chaotic Tools. It is purple, but shiny always emitting a creepy purple light.
Venomite Ore: Another ore, and more common than Chaos Gemstone. It is practically the Gold of the Shallows. It can be used to create Venomite Tools, and can use a Mithril Anvil at least. It is bright green, and the texture of Silver.
Nexus Crystal Ore: The absolute strongest, and most rare of the Shallows ore set. It is pure black, but lets out a red light every three seconds. Its tools can only be created on a Chaos Anvil. Its toolset is the absolute best you can find.
This is a name list. Find details on their individual pages.
Harvenger of Death, Eye of the Bone Catalyst, Onetheci (oh-neth-a-kai).