THCA is THC acid, the precursor to THC, the most abundant and intoxicating cannabinoid found in cannabis. When exposed to heat, non-intoxicating THCA converts to intoxicating THC and produces that classic cannabis high. Diamonds are another term for the crystalline structures that form in sauce, a cannabis concentrate with high levels of terpenes and a runny texture. Put two and two together and you get a term for the THCA-rich crystals that form in sauce. The resulting sauce can contain a mixture of both terpene-rich liquid and THCA diamonds. Further processing can also separate the THCA diamonds and refine them into crystalline THCA, a powdery concentrate comprised almost purely of the cannabinoid acid.