Siltstone is a very common natural block on the Stalactite layer underground - below the Fossil layer and above the Lava layer.
It can only be mined with at least an Obsidian Mining Cell or better.
Taking/mining a block of Siltstone is one of the unlocking requirements for the crafting recipes for Siltstone Wall and colored concrete building blocks.
On Siltstone Mirus and Warmworms can randomly spawn in absolute darkness, also Obsidian Treasure Chests can spawn occasionally. Still it is not easy to create artificials biomes/arenas from only Siltstone to spawn these creatures and/or chests if not on the Stalactite layer.
Siltstone can also be mined by using Super Excavators.