Stalactites are formed from the deposition of Calcium carbonate and other minerals, which is precipitated from mineralized water solutions. The corresponding formation on the floor underneath a stalactite is known as a Stalagmite. Given enough time, these formations can meet, resulting in a formation known as a column or pillar.
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda straw" stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws."
There are mnemonics to remember which formation hangs from the ceiling (stalactite) and which grows upward from the floor (stalagmite):
The longest stalactite known hangs in the Chamber of Rarities in the Gruta Rei do Mato (Sete Lagoas, Minas Gerais, Brazil). It is 20 metres long.
Icicles are not stalactites, since they are made of water.
Stalactites and stalagmites can also form on concrete ceilings and floors, although they form much more quickly there than in the natural cave environment.