The mined stones are classifield into different varieties.
01. Diamond, well crystallised, transparent and of gem quality.
02. Bort, boart or bortz, poorly crystallised, grey to brown in colour, translucent to opaque and used mostly in industry as an abrasive. The large and black stones of granular crystalline structure found almost exclusively in the State of Bahia in Brazil are also known carbonado or carbonate.
03.Ballas-spherical masses of minute diamond crystals more or less concentrically arranged. This mass is very hard and tough and has important industrial applications.
Rough diamond crystals are mostly of diameter less than a millimetre in diameter. Big crystals are extremely rare and so valuable that they acquire the status of an 'antique' and are known by special and distinctive names. History and legend play a bigger part than the actual inherent value of these stones. They are mostly found as part of the crown jewels of various nations.
A natural diamond mined from the earth always has inclusions of graphite or other metals that are visible as dark spots in the rough stone. Voids often filled with gas are visible as white or shiny spots within the stone. Stones that do not have any inclusions even under 50X magnifications are extremely rare and are collectors' items.
Carbon atoms arranged in inter-penetrating face-centred cubic lattices form the crystal structure of diamond. An inifinitely large crystal would be totally colourless and transparent except to ultraviolet light wavelengths shorter than 230 nm. However, all natural diamonds contain defects in the crystal lattice as well as impurity elements and these defects cause colour in the stone. Again, the growth patterns may abruptly change, with further layers becoming mirror images of the earlier growth. The 'twinning' that often occures within the crystal makes the crystals very hard to cut and they are called 'macles'.