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. Aggregates such as this form when two microdiamonds grow next to each other and merge. 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 with larger diamonds in the Earth's mantle (the layer beneath the crust), or in the rising magma (molten rock) that can bring the larger stones to the surface. Magnification: x93 at 6x7cm size. x160 at 4x5 inch size.