Yes, marble blocks. Blah blah blah. They've been suggested what, 10 billion times before? 10 squillion?
Marble blocks are an obvious addition for ornamental purposes, so I'm not going to just suggest adding marble because it looks pretty. Instead I'm going to suggest a specific practical purpose marble could have.
Marble blocks would be very tough, taking as long to break as obsidian. They could only be collected by iron and diamond picks.
Unlike obsidian though, marble would be prone to fracture and instantly break when exposed to an explosion. Ghast fireballs would be too weak to break them, but TNT and creepers would tear through marble as if it were dirt. Marble blocks are never destroyed by explosions though, only broken and reduced to collectible form.
Marble blocks would be slightly rare, but would usually be found together in large quantities, in quarries of 10ish-50ish blocks. These big lumps of marble could either be entirely underground or breach the surface.
A marble block with this property has a few purposes:
1) For security. You could create barriers that would take a very long time to break through, unless someone has TNT. It can prevent PvP opponents/griefers from instantly being able to cause major damage to your structure, until they take the time to collect enough gunpowder to make all the TNT necessary. At the same time TNT makes it easy for you to demolish and rebuild parts of your structure as necessary.
2) For challenge maps. You could have puzzles where someone has to figure out how to trigger TNT, or how to acquire TNT, to blast their way out of a marble dungeon or marble maze.
3) It would finally give TNT a purpose. Since TNT instantly breaks, but never destroys marble blocks, and marble is always found in large compact quantities, TNT would be the only practical way to mine marble.
Although a block besides marble could instead have the VERY TOUGH BUT VULNERABLE TO EXPLOSIONS property, I'd prefer marble for aesthetic reasons. We all want marble because it looks nice, might as well make it do something unique.
wow. a person can easily spam marbles like that. imagine in SMP:
Your walking along a forest when suddenly a random noob starts placing marble around you. as bad as obsidian but even EASIER to OBTAIN
it should be just a BIT more common than diamond. at the LEAST. and found in clusters of 2-5.
If it's as rare as diamond you can't really build much of anything will it. I want it to be common enough that people can build marble castles if they so choose.
It's still rare, I imagine it being as rare as coal. Maybe a little rarer. Except instead of being distributed in small chunks it's found more rarely, but in larger compact groups, ideal for blast mining. Just because you find it in larger bunches doesn't mean it's not rare, if you only find large bunches once in a while.
If some griefing noob puts marble around you, thanks noob for the free marble! It'd be like them giving you a valuable resource.