Ross Brighton, of Ilam School, asks :-

How many different kinds of stones are there in the world?

Alistair Campbell, a soil scientist with Lincoln University, responded.

There are two types of "stones" you may collect - rocks and minerals. Minerals are naturally occurring chemical elements or compounds of inorganic origin. For example, agate contains two chemical elements, silicon and oxygen, in the proportions of one to two hence has the chemical formula Si02.

There are about 3,000 different minerals. These differ from each other in many ways, including their colour, the shapes of their crystals and their hardness (you can scratch some with a copper coin, others need a pocket knife, and some not even with a file). Scientists can identify each by how they diffract X-rays.

Most rocks contain several different minerals. One example is greywacke sandstone which was first formed as sediments in the sea, fused with heat and pressure while underground and later raised up into mountains by geological movements. Rounded stones are produced when small pieces flake off the mountains and are tumbled down riverbeds. Other rocks, such as those formed in volcanoes, have cooled from a molten state. These are called igneous rocks.

Since there are probably at least as many different sorts of rocks as there are different minerals you would need to collect about 6000 to have a full collection. All major museums have extensive collections of minerals and rocks and usually have quite a few on public display.