A stalagmite is made of calcium carbonate, which is the same mineral that makes up limestone.

Here's why:

* Stalagmites form in caves due to the dripping of mineral-rich water.

* This water contains dissolved calcium carbonate.

* As the water evaporates, the calcium carbonate precipitates out and crystallizes, forming the stalagmite.

So, while stalagmites are not technically "limestone", they are made from the same mineral that forms limestone.