Although this stone sounds like a form of Turquoise, technically speaking it is in fact a teal-coloured speckled form of Jasper, which is in itself a member of the Chalcedony family of crystals.
It is mined exclusively in Africa and specimens vary considerably one from the other. One may be browner in colour and the next one a vibrant green in colour.
Although the basic colour maybe teal, African Turquoise is speckled with other coloured Jaspers as well.
Add to this the fact that inclusions of Copper and Iron are frequently present as well and you can understand why there are so many colour variations.
This "soup" of minerals ensures that, when polished, the surface patterns of the stone are wildly different one from another. No two pieces are ever the same !
However, most pieces that are mined are left as they are, simply polished and cut to reveal their lovely patterning.
Some of the more vibrant specimens though, the ones that are already leaning towards being blue/green in colour, are often chemically treated in order to make them look very similar to standard Turquoise - hence the name African Turquoise !