This was known as Stalactites and Stalagmites in the Legacy version, but is now known as Dripstone.
Dripstones are a variety of blocks that generate on the floors and ceilings of caves. Unlike their predecessors (see below), dripstones can be placed on walls and can be waterlogged.
Dying by falling onto a dripstone gives the death message:
Person was spiked by a stalagmite
There are currently 15 variants of dripstones: 14 for each type of stone (Stone, Obsidian, Andesite, Diorite, Granite, Sandstone, Red sandstone, White Sandstone, Chalk, Mordor Rock, Gondor Rock, Rohan Rock, Luigon and Cargon) as well as icicles.
There are also larger, two block tall forms of each variant that can generate as shown below. These generate as two dripstones on top of each other, which can be collected separately.
Dripstones are primarily decorative and can be found in many of the caves of Middle-earth. However, the upwards-facing forms have a secondary purpose as they deal damage to any creature (player and NPC alike) unfortunate enough to land on them. This property means that they are useful in trap-making to protect a base. Dripstones can be collected with any pickaxe.
In Renewed Snapshot 1.1, when dripstones were first ported to the renewed version, they were called 'speleothems'. This was changed in Renewed Snapshot 1.2 to the equally accurate but far more pronounceable 'dripstone'.
Stalactites and Stalagmites
Stalactites are blocks found underground hanging from the ceilings of caves. If they hang from a stone block, they will occasionally drip water.
Stalagmites are similar in appearance and physics to stalactites, but instead of facing downwards from the ceiling, they face upwards from the ground. These can usually be found in the caves under Middle-Earth, and can be collected with any type of pickaxe. However, be warned! Stepping on stalagmites will cause damage, and falling onto one causes even more damage, so be careful in caves!
Both types may be harvested and collected and are separate items. 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.
Ice and Obsidian variants can be found in the Ice and Obsidian levels of Utumno respectively.
Because of their ability to deal damage, stalagmites can be very useful for base protection and mob farming/grinding.