Cactus

Cactus is a block that is only available in Alpha mode. It was first seen in the Secret Friday Update 5, about Minecraft Alpha version 1.0.5_01. The block has one texture put on all sides of the block, one put on the top and bottom, and an unused texture resembling a tree crosscut indicating the inside of the cactus. It can be broken easily without any tools.

It appears on Sand in vertical of stacks that can be 1 - 3 blocks tall, 1x1 wide. They are somewhat rare, and appear in normal and winter themed maps. When the player touches the block, half a heart of damage is taken. Half a heart of damage is taken for each Cacti block walked on; for the player and mobs. This makes it useful for traps, such as a hallway with Cactus block floor. If any mobs to walk into the hall, the floor will kill them quickly. When mined, it can drop 1 cacti block.

It is possible that cactus blocks will serve more purposes in the future.

New cactus after the 23nd, July update.