Scale of Knoop
According to this scale, as according to all those which preceded it, the Diamant is the hardest material which one knows. That which comes in second rank is a Alliage of Carbone and Bore (extremely hard metal). Come then, in an decreasing order, the Topaze (Precious stone harder than the Acier with the Tungstène), the quartz, the Verre, and good the last, the Gypse which one makes the plaster.
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