Stalactites are blocks found underground hanging from the ceilings of caves. If they hang from a stone block, they will occasionally drip water. There are also stalagmites, which are similar to stalactites, but point upwards from the ground instead of downwards.
Both types may be harvested and collected. Stalactites must be placed below a solid block and stalagmites placed above a solid block, but if the surroundings change and these placement conditions are no longer met the block will drop as an item. Otherwise, they may be harvested with any pickaxe.
They currently have no use in gameplay, except for making traps on servers, for decoration or for grinders for npcs such as Orcs.