The purpose of petrology is to understand the conditions of the formation of rocks. The first stage of this approach is to describe and classify the rocks; that is the subject of petrography. Many characters can be used: density, hardness, colour, structure (on the scale of the outcrop), texture (on the scale of the sample or of the microscope), mineralogical composition, chemical composition, speed of propagation of the waves, etc. Among all the characters, the mineralogical composition is certainly the most important: it allows us to, more or less, accurately foresee the other characters and it can be used to determine the very conditions of the formation of rocks. Indeed, the nature and chemical composition of the minerals in a rock obey stricter laws, since the formation temperature was higher

Minerals, both metallic and nonmetallic, and building stones have played key role in shaping human civilization since ancient times. Demand for and production of these earth resources increased manifold during the Industrial Revolution and both have seen an exponential growth during the last century, keeping pace with growing national and global economies. Ever-widening base of mineral use has made minerals an integral part of our everyday life.

Although Britain is a major gemstone trader, it currently produces little from indigenous sources. There are, however, records of a wide variety of gem material, particularly in Scotland. In addition to sapphire, ruby and possible diamond, the country has yielded topaz, beryl, and many varieties of semi-precious stones including cairngorm, amethyst, garnet, tourmaline, agate, zircon, ‘Blue John’ flourite and jet. There is also an unconfirmed report of Scottish emerald. Most of these gemstones were found by amateur geologists and gemmologists but recent discoveries, particularly of iamond indicator minerals in Scandinavia and Ireland, have raised commercial interest in the potential of Britain as a source of precious stones. <...>0