The Diamond Chest is an upgrade from the Gold Chest, both of which are added by the Iron Chests mod. It has 108 slots, which is 4 more than the Alchemical Chest (104), 27 more than the Gold Chest (81), and 81 more than a normal Chest (27).
This chest can be upgraded to a Crystal Chest, by crafting or a Chest Upgrade. However it is highly advised not doing this since the Crystal Chest tends to create a lot of lag, while still having the same amount of slots.
34886 EMC. This price does not assume crafting the Gold Chest is made by the most efficient means possible, which is Chest > Copper Chest > Iron Chest > Silver Chest > Gold Chest. Turning that Gold Chest into a Diamond Chest would have an aggregate cost of 24810 EMC.
The crafting recipe does not show up in the NEI, as with all the other chest types in Iron Chests.
While using inventory options from NEI, the chest may appear open forever or until it is broken or the world is reloaded.