Cave formations are called speleothem, from the Greek word "spelaion", cave and "thema" meaning deposit. Almost all of them are made of calcite, the crystal of calcium carbonate. Different minerals and different movements in the water account for an incredible variety of cave formations.

Successive rings of calcite crystals dripping slowly from the ceiling form a tiny tube, a soda straw.

As water saturated with calcium carbonate drips through the ceiling of a cave, successive rings of calcite crystals form a tiny tube, a soda straw.

Water flows down the outside of a filled soda straw and the calcite deposits thicken to become a stalactite.

If the tube fills, water flows down the outside and the calcite deposits thicken to become a stalactite.

In this photo of a stalactite that broke off, you can see how the calcite flowed in rings.
In this photo of a stalactite that broke off, you can see how the calcite flowed in rings.
In this photo of the bottom of a stalagmite you can see how it formed on the stones in the underground streambed.
In this photo of the bottom of a stalagmite you can see how it formed on the stones in the underground streambed.

Water dripping onto the floor builds stalagmites, which in time may join stalactites to form a column.

Water dripping onto the floor builds stalagmites.

Water drizzling along a slanted ceiling builds delicate sheets of rock, called cave draperies or curtains.

Water drizzling along a slanted ceiling builds delicate sheets of rock, called cave draperies or curtains.

The deposits begin as winding trails of calcite, which later extend downward and thicken.

The deposits begin as winding trails of calcite, which later extend downward and thicken.

The white portions are almost pure calcite, while other minerals, mostly iron, make the orange and red stains.

The white portions are almost pure calcite, while other minerals, mostly iron, make the orange and red stains.

When water seeps through tiny fissures in the bedrock, it can build incredible unlikely formations called helictites. Water is forced up its tiny central tub, forming a crystal at the tip.

Other speleothem grow on cave pools. Thin layers of calcite forms on the surface of pools as they evaporate. As they float, more and more calcite attaches to it in circular patterns.

Rimstone that is left after the water has seeped to a lower level of the cave.

These "cave rafts" continue to float on the surface of the water as long as it is undisturbed until they reach a weight that pulls them under.

Deposits called shelfstone form where the surface of the water touches the sides of the cavern or submerged columns.

Thicker deposits, called shelfstone form where the surface of the water touches the sides of the cavern or submerged columns.

In shallow pools fed by dripping water, cave pearls may form. These have joined together when the pool they were in was no longer in as much motion.
In shallow pools fed by dripping water, cave pearls may form.
These have joined together when the pool they were in was no longer in as much motion.

In shallow pools fed by dripping water, cave pearls may form. These small spheres are produced by layers of calcite forming around a grain of sand, much like oysters make the pearls they are named after. The agitation of the dripping water makes the pearls almost round and keeps them from sticking to one another or to the bottom of the pool.

Flowerlike structures can grow. Crystals of calcium sulfate percolate through porous rock. As new crystals are formed from behind, the flower may grow in a variety of shapes.

Large crystals may form in underwater chambers.
Large crystals may form in underwater chambers.

Large crystals may form in underwater chambers.

Many cave walls are made of onyx, a colored variety of calcite with the crystals so tiny you can't see them (microcrystalline).
Many cave walls are made of onyx, a colored variety of calcite with the crystals so tiny you can't see them (microcrystalline).