Diamond Treasure Chests can be found in the Corruption Layer, which is currently (R23 in October 2015) the deepest level of any Creativerse-world.
Diamond Treasure Chests will also spawn/appear at any height in Creativerse-worlds on dark arenas with floors made of corrupted blocks - as long as enough corrupted blocks have been placed in one chunk (area) of the gameworld. (One chunk is 16x16x16 blocks large.)
These treasure chests will only spawn in complete darkness, so adding roofs and walls to any spawning arena/farm will be of benefit, otherwise these chests can merely spawn at night.
Artificial light like torches or lanterns will hinder chests from spawning. Blocks like Hardened Lava or Wildwood Flowers can be used to have a little light though. Also sunlight or artificial light will not make chests vanish that have already spawned.
As of R25 (November 2015) Diamond Treasure Chests can contain:
Iron Mining Cells (rarely),
Iron Swords (rarely),
Iron Armor (not often),
Arctek lighting (rarely),
Arctek tables (rarely),
different Arctek chests (rarely),
- and a range of rare recipes for fancy building blocks (decor), roofs and colored beacons.
Unlooted treasure chests can and will despawn when (all) player characters leave out of sight (and/or log out). Half-looted chests will prevail a little longer, but will despawn if the player/s do/es not return quickly.