Inclusions of fossil plant debris in opal from Wollo, Ethiopia
Partially etched diamond crystal
Inclusion of apatite in ruby from Tanzania
Inclusions of anatase crystal and rutile fibres in rock crystal from Brazil
Inclusions of hematite in feldspar (sunstone), India
Inclusions of rutile (dark crystal and fine needles) and clusters of zircon grains in umbalite garnet, Tanzania
Midge included in amber from the Baltic coast, polarised light
Inclusions of hematite in aquamarine, Tanzania

Where does Colour come from?

Research on the causes of colour in gemstones is one of the most fascinating aspects of Gemmology. It combines areas of optics and chemistry with the latest findings on the structure and forces of atoms.

Coloured stones are divided into three categories:

Stones, in which the colouring element is an integral part of the mineral, are called idiochromatic. These gemstones therefore only appear in one colour and include stones such as turquoise, peridot and rhodonite.

Most gemstones would yield colourless crystals with a pure chemical composition. They obtain their colour from a small addition of colour-imparting elements; they are therefore called allochromatic.

So where does colour come from? Colour often comes from the random presence of other colouring elements in the crystal lattice. These extraneous atoms of titanium, chromium, iron, nickel, vanadium, manganese, cobalt, and copper provoke the amazing phenomenon of colour. Sapphire, ruby, emerald, tourmaline and many others all belong to the allochromatic category of colour. Even more astonishing is that depending on the host structure of the stone; the same chemical element can produce different colours, for example chromium gives ruby ​​its element of purple, yet in emerald is gives an inimitable green.

Another means of colouration can be found in the fascinating charm of the colour of opal, which lies in its extraordinary play of colour due to the scattering of light off of its crystal structure.