What is Stone?

What is stone? The Oxford English Dictionary defines stone as "A piece of rock or hard mineral substance (other than metal), of small to moderate size". One of the most commonly used terms related to geology it derives from stan (Old English) and steinn (Old Norse). In the Gaelic it is clach.

The formation of sedimentary rocks was described in 1802, by Professor John Playfair, uncle to architect William, and friend of James Hutton, founder of modern geology. The notable 19th century Scottish geologist Sir Archibald Geikie eloquently commented on the process of transition from unconsolidated sediment to sedimentary rock: "If you take a quantity of mud, and place it under a weight which will squeeze the water out of it, you will find that it gets firmer. You can thus harden it by pressure. Again, if you place some sand under water which has been saturated with lime or iron, or with some other mineral that can be dissolved in water, you will notice that as the water slowly evaporates it deposits its dissolved material round the grains of sand and binds them together. Were you to continue this process long enough, adding more of the same kind of water as evaporation went on, you would convert the loose sand into a solid stone."

This is essentially the process of lithification (derived from the Greek word lithos – a rock). Thus in a sedimentary basin through compaction by the weight of overlying strata and by cementation by mineralised fluids so sedimentary rocks are formed. In contrast, igneous rocks are usually consolidated by crystallization of minerals from molten material either within the earth’s crust (e.g. granites) or at the surface (e.g. basalt lavas). As their name suggests metamorphic rocks have been transformed (recrystallised) from the original rock by a combination of heat and pressure. In these circumstances, new minerals sometimes form.