Carbon flaw
Encyclopedia
A carbon flaw is a blemish present within a diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

crystalline form of carbon, usually seen as a black spot. The blemish may be microscopic or visible to the naked eye.
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