Kaolin

Kaolin is a bright white, very fine, mineral-rich clay that naturally occurs as a result of the decomposition of feldspar in granite deposits. It is often found in tropical climes and the name derives from the name of the Chinese mountain where it is supposed to have been first dug up, but high concentrations of good quality kaolin can also be found in the UK, mainly in the South-West. This particular sample was found in Cornwall, where unlike tin and copper mining, kaolin extraction and processing are still very active. 

Although it is now used for a huge variety of applications, for example as a polishing agent in toothpaste and as a light-diffusing material in incandescent bulbs, kaolin originally came to prominence as a key ingredient in the production of porcelain. Although European travellers in China encountered porcelain from as early the 1100s, their attempts to imitate Chinese porcelain production were fairly unsuccessful. They ended up producing something called 'soft paste' which was not really 'true' porcelain as it didn't have the hard, glass-like qualities you get from firing kaolin clay at high temperatures. Eventually in the mid-18th century it was recognised that the production of vitrified, translucent, 'hard-paste' porcelain similar to the Chinese material. 

Whilst it's still important to ceramicists, kaolin is now largely used in the paper industry as a filler between the paper fibres and as a coating to enhance the colour, brightness and glossiness of paper. Kaolin can contain slightly elevated levels of uranium and thorium; whilst you can't detect it in a single glossy magazine, you may find that a truck-load of them will trip a radiation monitor.