[ericfranklin]

The testers a typical jeweler or diamond merchant might use only identify between diamond, cubic zirconia or moissanite. They do not identify origin of a diamond.

The testers that can identify a diamond's origin are quite expensive and usually only found at major gemological labs.

All synthetic white diamonds are "Type IIa" (no or trace amounts of nitrogen) while only 1-2% of mined diamonds are IIa. 97% of mined diamonds are "Type Ia" (clusters of nitrogen). It is possible that is what they were referring to about purity, but the amount of nitrogen is not a definitive indicator of origin.