My take on this proposition.
Diamonds are a natural kind.
Cattle are the artifactual kind. Cutting up the diamond is just like slaughtering the cow, it is something we do to make some object suit a certain purpose. The relativity in how well or poorly the diamond is cut and the relativity in how to the point is the killing of the cow lead to vagueness of each artifactual kind in turn. The kinds that are diamonds and cows are not similarly vague, though they may be vague for other reasons.