This series of articles briefly examines how and where gems originate.  

In the case of diamond the long-standing mystery of its source was finally thought to have been resolved in 1870 with the first discovery of a diamond bearing kimberlite in the South African Free-State. 

However modern research has revealed that kimberlite is not the matrix in which diamonds formed. In fact diamonds are formed in the very active high pressure and high-temperature zone that lies between the Earth’s solid crust and its outer mantle (Fig 3). That said, it should be noted that the occurrence of diamond is rarely associated with typical volcanic activity.

This is because at the temperatures that are attained by most volcanic lavas diamond would burn in an instant. 

However kimberlites and their close relatives lamproites are largely composed of olivine/peridotite and these minerals form magmas that flow at relatively low temperatures. Therefore bodies of this nature are eminently suited to act as transport mediums capable of carrying diamond to the surface from depths in excess of ninety miles.

Kimberlite and lamproite rocks have distinct similarities in composition but they differ widely in form. Kimberlite bodies have a “carrot-like” shape (Fig 4) whereas the profile of a lamproite (eg Argyle, Australia) bears a much closer resemblance to that of a mushroom (Fig 5). 

Eclogites (Fig 6) are fascinating undigested nodules of ultra-mafic rock that are occasionally found, like raisins in a pudding, as a component of certain kimberlite bodies. Such specimens provide science with a great deal of information about the nature of our Earth at depth. They frequently contain euhedral crystals of garnet, chalco-pyrite, olivine and diamond (Figs 1 & 6).    

Alluvial and elluvial deposits were worked as major sources of diamond for many centuries, firstly in India, then Brazil and eventually in Africa. While elluvial deposits are those formed by the weathering of the diamondiferous magma in situ, alluvial deposits are those that have been created by the actions of moving water. 

Changes in the direction and speed of flow of the water course can result in the creation of concentrations of heavy minerals in placer pockets. A knowledge of these factors facilitates the recovery of diamonds from modern rivers (Fig 10, 11 & 12) and this is equally true in the case of the buried paleo-channels that were left by long dead rivers (Figs 13 & 14). 

Skilled prospectors trace the course of these ancient river beds to find and work the diamondiferous gravels as dry diggings. It is fascinating to learn from these same diggers that there are times when they may dig down to a river bed only to discover the presence of a hearth. When they see this they immediately realise that the site has been worked by stone-age men looking for diamonds. 

Primitive man valued diamonds for use as etching stones to engrave their artwork on hard rocks. The diggers know the bush-men must have had sharp eyes and that there will be no diamonds left so they just close up and move on to another site. Garnet is a common feature in the heavy concentrate from alluvial diamond diggings. 

Chrome-pyrope garnet, in particular, is employed as an important indicator