Microdiamond. Coloured scanning electron microg- raph (SEM) of a microdiamond crystal from Siberia, Russia. Microdiamonds are diamonds (pure crystals of carbon) which measure less than half a millimetre in any dimension. Cubic crystals such as this are found in carbon-rich areas only. Microdiamonds are being studied as an alternative to the more expensive larger diamonds used in industry, for instance as heat dissipators or abrasives. They are thought to form either with larger diamonds in the Earth's mantle (the layer beneath the crust), or in the ascending magma (molten rock) that can bring the larger stones to the surface. Magnification: x114 at 6x7cm size. x190 at 4x5 inch size.