Opal is an amorphous mineral with a water content of 5-10%. It is formed when colloidal silica gel is deposited slowly at a low temperature. It never appears in crystal form but in veins, nodules and crusts with wonderfully iridescent colours, produced by the diffraction of light on silica spheres.
The ruby is a variety of corundum and derives its name from the Latin ‘rubeus’, meaning red. The term ruby first appeared only in the Late Middle Ages. Before that the stone was known in Italian as “carbonchio”, small piece of coal, on account of its colour, reminiscent of burning embers. Its typical inclusions are very fine rutile needles that intersect to form a lattice known as ‘silk’.
Meteorites are formed in the solar system, mainly in the asteroid belt between Mars and Jupiter. When they hit each other they generate fragments, which after a long journey may pass close to the Earth and be attracted by its force of gravity.
Uvarovite is the rarest variety of garnet and, due to the chromite it contains, is a brilliant emerald green in colour. It was named after the Russian count and statesman Sergey Uvarov Semeonovich, a great lover and collector of minerals, by Germain Henri Hess, the man who discovered it.
Amber is a fossil resin of very ancient origin and has been used since the Stone Age. It may contain insects and plant material imprisoned at the moment of its formation.
Tanzanite is the blue- or violet-coloured gemmological variety of the zoisite family and comes from Tanzania. It was discovered in early January 1967 by a certain Mr Ngoma in the Arusha region of the country.