long long ago I used railcrafts crushed obsidian as IC2 reactor containment.
it has a really high blast resistance (2 layers are sufficient for a reactor) and it can be easily removed with a shovel, but it's hard to remove with a pickaxe. of course you need something else as roof (or at least one layer of something not burnable)
what's the blast resistance of gravel ores?
by the way, there is a bug (or crossmod-exploit) regarding the reactor: somebody at IC had that fancy idea that people should not use warded stone and other indestructible blocks as casing and they tweaked the system to melt indestructible blocks. this way, one could easily melt a hole into the nether-roof, or if someone has a base covered in warded stone... just melt a hole into that wall.