Microdiamond. Coloured scanning electron microg- raph (SEM) of the etched surface of a microdiamond crystal from Siberia, Russia. Chemical erosion has caused the surface to break up. Microdiamonds are diamonds (pure crystals of carbon) which measure less than a millimetre in any dimension. They are being studied as an alternative to the more expensive larger diamonds used in industry, for instance as heat dissipators or abrasives. Micro- diamonds 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: x350 at 6x7cm. x575 at 4x5 inch size.