What is a stone that burns?

Well, sulphur...
although coal is a mineral (hence stone) and also burns...

And, technically, I guess, pure sodium (that which burns) is a metal.

Potterdude gets the golden banana with sulphur.

http://www.chemtutor.com/elem.htm#sulfur


Sulfur burns in air (the stone that burns) to form sulfur dioxide. This is the first step in the manufacture of sulfuric acid, by far the most used compound of sulfur. It has been said that the amount of sulfuric acid made is a good measure of the level of industrialization of a country. Sulfur is one of the main ingredients in the vulcanization of rubber.

Sulphur isn't really a stone. It's crumbly yellow, and is essentially an element. It also forms bright yellow crystals which do not burn.

Apparently sulphur is well known as 'the stone that burns'. Alchemy aims at the great human "goods": Wealth, longevity, and immortality. It was believed that metals possessed magical powers in their alloy forms, specially "mercury", the liquid metal, and "sulfur", the stone that burns.

And...

https://cdn.preterhuman.net/texts/other/crystalinks/alchemy.html

Sulfur (brimstone, stone that burns) reacts with O2 giving a blue flame:

S + O2 = SO2

That doesn't change the fact that it's not a 'stone', it's an element. Dictionary.com defines a stone as 'a small piece of rock', and defines rock as 'a relatively hard piece of matter that is naturally formed'. Although sulphur is 'natually formed', it is not 'relatively hard'. It crumbles very easily.

Besides, the 'stone that burns' is a nickname, given to it by people in the smelting industry.

Yes, stone is made of rock and geologically, rock is "any natural material, hard or soft (e.g. clay) consisting of one or more minerals." Source: the OED.

Face it, Flynn. Sulphur is an element and is also a mineral that is often found as lumps of yellow rock or stones (brimstones, in fact).