The usual explanation is that water drips, evaporates, and leaves minerals behind. That is not what happens. Caves are close to 100 percent humidity, so almost nothing evaporates. Stalactites grow by a different mechanism entirely, and it runs on carbon dioxide.
The chemistry, in three steps
Rainwater picks up carbon dioxide from the atmosphere and, far more importantly, from soil, where root respiration and microbial decay can push CO2 concentrations to many times atmospheric levels. Dissolved CO2 makes the water weakly acidic as carbonic acid.
That acidic water percolates into limestone and dissolves calcium carbonate, carrying it in solution as calcium bicarbonate. This is the process that makes caves in the first place, and it is the engine behind karst landscapes, sinkholes and underground rivers.
Then the water emerges into the open air of a cave chamber, where CO2 concentration is much lower than it was in the soil. Dissolved carbon dioxide degasses out of the water, the water becomes less acidic, and it can no longer hold as much calcium carbonate in solution. Calcite precipitates. Not from drying, from degassing.
Why a stalactite is hollow to begin with
A new stalactite starts as a soda straw: a thin, hollow tube exactly the diameter of a water droplet, growing downward at its rim as each drop deposits a tiny ring of calcite before falling. Soda straws can reach several metres and are extremely fragile.
When the central tube blocks, water starts running down the outside instead, and the straw thickens into the conical shape most people picture. Whatever falls to the floor builds a stalagmite, blunter and broader because the water splashes.
Reading the bands
Slice a stalagmite and you get concentric growth layers, often annual, that respond to how much water was moving and how much CO2 the soil above was producing. Both depend on rainfall and temperature.
That makes speleothems a genuine climate archive. They can be dated precisely using uranium-thorium methods, and their oxygen isotope ratios track the isotopic composition of rainfall. Stalagmite records have supplied some of the best-resolved continental climate reconstructions going back hundreds of thousands of years. The general principle of what rhythmic layering records is covered in banding in mineral growth.
Flowstone, travertine and tufa
Not all carbonate deposition happens on a ceiling. Where water sheets across a surface it builds flowstone. Where CO2-rich groundwater emerges at a spring it builds travertine, the banded, often warm-toned carbonate that the Romans quarried by the millions of tonnes for the Colosseum.
New York has its own version of this, and our travertine from Ilion Gorge is exactly that material, discussed at length in travertine at Ilion Gorge. It is the same chemistry as a stalactite, happening outdoors.
Cave calcite versus vein calcite
Speleothem calcite is usually banded, fibrous and massive rather than well-formed crystals, because it is precipitating fast from thin films of water. Calcite grown slowly in an open cavity from circulating fluids produces the sharp crystals collectors want, like our golden calcite point and the Weston lot. Both are calcite. The texture is a record of growth rate, a point made more generally in crystal habit.
One thing not to do
Do not break formations in caves, and do not buy speleothem material. Speleothems grow at rates measured in fractions of a millimetre per year, they are irreplaceable on any human timescale, and removing them is illegal in most protected caves. Touching them with bare hands transfers oils that can stop growth at that spot permanently.
Admire them where they are. There is plenty of legitimately collected calcite, and it grew by the same beautiful mechanism.
