Dismissive analysis of jewel on the counter that shows explosive power (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

NOT A GEM (perhaps a declaration that a supposed gemstone is not genuine = dismissive analysis of jewel), reversed (on the counter). Short for “megaton TNT equivalent” = a measure of explosive power equivalent to the detonation of 1 million tons of TNT.