Crafting Recipe

GGG
RCR
GHG

G = gunpowder; R = redstone; C = Crystal; H = creeper head

Ability

To use the Explosive Crystal, right-click it when it is in your hand. It will summon 10x TNT with a fuse of 200 ticks (10 seconds). Every time you use this crystal, there is a 1.5% chance of it breaking, so it can become very overpowered when used more than once (don't hold down right-click!).

Obtainability

Possibly the rarest item required to craft the Explosive Crystal is the creeper head. To get this, there are only a couple options. The first is you wait for lightning to strike a creeper, then get the supercharged creeper to blow up another creeper and get its head. Or, you could enchant a trident with Channeling, in which case you will still have to wait for a thunderstorm, but you could strike the creeper at your own will.

Strategy

Because the TNT has a long fuse (200 ticks or 10 seconds), it may not be easy to use it to kill mobs that are chasing you. However, it is certainly helpful for destroying structures or mobs when they are trapped in a location. You could also use to mine, as long as you're very careful (and maybe have a piece of Blast Protection armor).