Chain

Chains are blocks found in bastions, it only can be placed in the ceilings of non-transparent blocks, it also can be connected with a lantern in the bottom of the chain. It's obtainable by placing iron nuggets on the top and the bottom, and put iron ingot in the middle.

Natural generation
They generate in bastion remnants.

From block loot
Chains can be mined using any pickaxe. If mined without a pickaxe, it probably drops nothing, but this hasn't been tested.

From crafting
Can be crafted from iron nuggets on the top and bottom and iron ingot in the middle of the iron nuggets.