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Alleged "diamonds" turned yellow with heat

Here is my problem. A customer sent in some old jewelry to be

One ring had about 20 little 2 mm round stones that two of my
employees both tested as diamonds. Another employee took the shortcut
of melting the ring to remove the stones. (We do this quite often,
usually for material we are scrapping out.) So the stones turned
various shades of yellow and tea-orange. Actually quite attractive
colors. But they don’t test as diamonds anymore. The tester we use is
a Gemoro Ultra-tester II.

Obviously I have to tell my customer what happened. It would be
useful, and make me seem not quite so stupid, if I had some idea what
material would fool the diamond tester and also turn yellow with

Any ideas?

Several days have passed since I posted this. The problem is
resolved. I learned from a graduate gemologist that smaller
Moissanite are more likely for fool the gem tester and that yes they
do turn yellow or brown when heated.

It turns out that the customer already suspected that some of the
diamonds (there were multiple rings) were not real. He was not the
least bit upset.

The new job is finished. I am going on vacation. Life is good.

Steve Walker