Just like Grass spreading on Dirt blocks, there should be other blocks that can spread on other blocks.
Mossy Cobblestone can turn other Cobblestone and Stone Bricks into mossy ones SO LONG AS... There is a light level of 7 or lower and water up to 2 blocks away from the Mossy Cobblestone or Stone Brick and the Cobblestone, and the biome is a wat biome.
>Mossy Stone Brick
Mossy Stone Bricks can turn other Cobblestone and Stone Bricks into mossy ones SO LONG AS... There There is a light level of 7 or lower and water up to 2 blocks away from the blocks being affected and the mossy block itself, and the biome is a wet biome.
Nether Bricks can turn other Brick blocks into Nether Brick SO LONG AS... There is lava one block near the Brick and the Nether Brick blocks.
>Nether Brick Stairs
Nether Brick Stairs can turn other Brick Stair blocks into Nether Brick Stairs SO LONG AS... There is lava one block near the Brick Stair and the Nether Brick Stair blocks.
Lily Pads can create more Lily Pads SO LONG AS... The block where the new Lily Pad will be created is a water source block, There are no more than 5 lily pads within a 9 block radius.
Tall Grass can create more Tall Grass SO LONG AS... The block where the new Tall Grass will be created is a Grass Block, and there are no Snow Layers nearby, and there are no more than 25 within a 9 block radius (however, bone meal and natural generation can make as much tall grass in an area as much as possible).
But what would stop the moss from running rampant? What if moss only spread to adjacent blocks, and only when, say, the faces of the mossy and clean blocks both have an unobstructed view of the sky/sun (facing east or west) on days after it rained?
How's that for overcomplicating things? Anyway that should satisfy those guys who insist that it "should be rare," as well as being an intense learning experience for whoever would have to write the code for it.
Now that apples can be found anywhere, are they rare and valuable anymore? No.
Same with mossy cobble, if it is just farmed excessively, it is no longer rare, it loses its touch.
Making mossy cobble renewable would be like making diamond renewable. It wouldn't be a big deal if you have some, because you can make an infinite amount of it.
I guess I can't wrap my head around the whole "rare" thing.
In survival, I leave the stuff lie. In creative, well, I have as much as I want. It's not that pleasing (unless you have a texture pack that completely changes it) and don't care much to use it in simple survival construction.