The GEM DiamondMaster is one of a number of thermal testing instruments introduced specifically to separate diamond from its simulants. Although originally reported to measure thermal conductivity, these instruments in fact measure the thermal inertia of the material being tested. This article explains thermal properties in general and thermal inertia in particular, and then describes the limitations and potential applications of thermal testing instruments such as the DiamondMaster. Included are suggestions for the use of the DiamondMaster to separate various colored stones. -- AATA