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Discover stone;Discovery series

It's the stuff of great megalithic monuments; we love to hew and shape it, but we say 'stone cold' when it can be warm to the touch. In the third of our Discovery series, Gerald Haigh digs deep into a natural material we have grown up with

We seem to be rather ambivalent about stone. On the one hand, our language is peppered with such expressions as "stone cold", "stone dead", "heart of stone", that suggest we find it lifeless and unappealing. And yet the human race is comfortable with stone. We have grown up with it. The first things we ever made - ornaments, tools, weapons - were all of stone.

Megalithic monuments, the temples and pyramids of Egypt, the Easter Island statues, the great cathedrals of the world, all demonstrate our continuing determination to hew, shape, drag and erect massive stone witnesses to our existence.

Go to the stoneyard at York Minster and stand before a five-ton block of magnesian limestone, already many millions of years old, now brought into the light and destined for public immortality on the face of the building. Enjoy the way the sunlight provides contrasts between its faces. Place your hand on it and what you feel is not a cold unforgiving surface but warmth that feeds back into your hand, with the reassurance of tradition and continuity.

Stone is rock that has been extracted from its natural setting and used for something. Rock is formed by heat and pressure within the Earth's crust. There are three basic sorts:

* Igneous These were formed when molten matter from the Earth's core cooled and became crystalline and solid. Granite is an igneous rock.

* Sedimentary These are formed when particles of material eroded from older rocks by water, wind or glaciation, are laid down in beds and solidified by pressure. Sedimentary rock often contains organic material - fossils, plants, minute sea creatures. Limestone and sandstone are sedimentary rocks.

* Metamorphic This is sedimentary rock that has been changed in character by extreme heat and pressure under the surface of the Earth. Marble is a metamorphic rock formed from limestone. Slate is a metamorphic rock formed from clay.

Each type of rock has characteristics that can make it useful for a particular purpose. Marble, for example, can be white when pure - although impurities give it a distinctive colour and attractive markings. It can be easily sculpted or polished, or used for building. Michelangelo's statue of David and the Taj Mahal are both made of marble.

Good quality limestone is durable and strong and can be worked into complicated shapes. Slate can be split into thin sheets, making it an excellent roofing material.

Before the age of mechanised transport, stone used for building was always taken from rock near to the place it was to be used. Much of the stone for York Minster, for example, was quarried at nearby Tadcaster. Often, though, builders went far afield to places which have become famous for stone of high quality. The stone for Norwich Cathedral came in medieval times from Caen, in Normandy, even though the transport costs quadrupled its price.

The quarries of Carrara in Italy still produce the beautiful white marble which was first used by the Romans and later by the great Italian sculptors, including Michelangelo.

In the United Kingdom the most famous high-quality building stone has come for hundreds of years from quarries at Portland in Dorset. The raw material of many of London's public buildings has come from this tiny peninsula - the Wren churches (including St Paul's), the Law Courts in the Strand, practically the whole of Whitehall and Trafalgar Square.

Now with the quarries much less active than they once were, the Portland Sculpture Trust brings people from all walks of life to work with tutors in one of the quarries, expressing their ideas with traditional tools on Portland stone.

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