Decay (Mystcraft)

Decay is a mechanic in Mystcraft where extremely unstable Ages "decay" over time; blocks in the Age gradually get overwritten by blocks of decay, eventually making the Age unplayable.

Blocks of Decay (excluding White Decay) will spawn between Y=25 and the highest block naturally generated by the Age, including floating islands or tendrils. All types (excluding Black Decay) can spread through Air.

Another effect of Decay is the "crumbling" of an Age: blocks will degrade into lower tiers of blocks. For example, Diamond Ore will degrade into Coal Ore, or Stone will degrade into Cobblestone. Depending of the severity of the instability, Bedrock may also degrade, leaving a void world.

Black Decay
Black Decay is the slowest type of decay, slowly spreading at a constant rate, only through blocks. If 2 blocks of Black Decay form on top of each other, they will merge into one and force all blocks above them to obey gravity and fall, even if they wouldn't normally. This includes Bedrock. This means that Black Decay is the most insidiously destructive form of decay, in that it may not be visible in the world until holes start appearing that lead to the void.

It can be destroyed with TNT or mined quickly (~1 second) with a Diamond Pickaxe. It will never drop as an item.

Blue Decay
Blue Decay is faster then Black Decay; the speed depends on the hardness of the block it is spreading to: the faster it is to mine, the faster it will spread. As a side effect, Blue Decay will never spread to Bedrock or other unbreakable blocks.

It can be destroyed with TNT or mined quickly (~1 second) with a Diamond Pickaxe. It will never drop as an item.

Red Decay
Red Decay is about as fast as Blue Decay; the speed depends on the explosion resistance of the block it is spreading to: the weaker it is to blow up, the faster it will spread. As a side effect, Blue Decay will never spread to Bedrock or other unbreakable blocks.

It can be destroyed with TNT or mined quickly (~1.5 seconds) with a Diamond Pickaxe. It will never drop as an item.

Purple Decay
Purple Decay is a severe form of decay, quickly spreading through liquids, other decay blocks, and Air. While it can spread via blocks, it is slower then Red or Blue Decay.

It can not be destroyed with TNT, but can be mined with a Diamond Pickaxe, albeit very slowly (~9.5 seconds). It will never drop as an item.

White Decay
White Decay is the most severe form of decay, quickly spreading and destroying through any medium (even other decay blocks). Unlike the other types of decay, White Decay will deal a half-heart of damage per second to any player moving on it; this ignores armor entirely.

It can not be destroyed with TNT, but can be mined with a Diamond Pickaxe, albeit very slowly (~9.5 seconds). It will never drop as an item.

Unlike other decay blocks, White Decay will spawn from Y=20 to the highest block in the Age, naturally generated or not.