Sometimes confused with the visual size of stones, it refers to weight only. The special unit of measurement used to weigh diamonds and gemstones is called ‘carat’. Sometimes confused with the visual size of stones, it refers to weight only. The size does impact different stones’ measurements though. Shapes and types of gemstones can show slightly different readings due to variable densities. For instance, a 1.00 carat round sapphire might measure 6.0 mm, yet a 1.00 carat round diamond will measure 6.5mm. T.C.W. stands for ‘Total Carat Weight’, describing the weight of all gemstones or diamonds in a jewellery item - when there are more than one - in total. Earrings featuring diamond solitaires, for instance, are commonly cited in T.C.W. – the combined weight of both stones being indicated.