Diamonds - or morecorrectly diamond particles or grit - are used in the construction of diamond wheels, which are used to generate lens surfaces before smoothing and polishing and also to edge the finished lens to shape. They are also used to generate the curves on the glass blanks used for casting lens blanks produced from optical resins. The reason for the virtually universal use of diamonds for these tasks is, of course, the extreme hardness of the material. Diamond is the hardest known mineral, with a value of 10 on Mohs' scale (the scale for determining the comparative hardness of minerals and the resistance which it offers to abrasion). On this scale Talc is given the value 1 and diamond 10 - each mineral in the scale will scratch one with a lower value, but in turn will be scratched by those with a higher value. Another scale is the Knoop scale of comparative hardness.
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