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The weight of a diamond is measured in carats with 1 carat measuring 200mg.
Generally, the larger the carat weight the bigger the diamond.

The scale of colour runs from D-Z with D being the most white and
colourless and Z being tinted yellow/brown.

The clarity grading of a diamond refers to the number of imperfections and inclusions within the diamond.
Most grades require magnification for these to be visible to the naked eye.