In the context of cannabis extracts, diamonds does not mean the precious gemstone, although the substance is no less prized among cannabis fanciers. Diamonds refers to the crystalline deposits within a cannabis extract, as in the kind that would be used for dabbing.
Diamonds are a sign of pure THC-A
THC-A is the chemical precursor to THC, the psychoactive compound of cannabis. THC-A must be transformed or decarboxylated into THC, typically by applying heat. The diamonds in a THC extract are crystallized lumps of THC-A. The diamond crystals in an extract have an appearance just like any natural crystal such as salt, sugar, or ice. By themselves, they are colorless, but typically they appear swimming in sauce or distillate oil, which will give them the range of honey-like tones found in typical extracts.
In a nugget of dried flower, one may also sometimes observe these crystals encrusting the trichomes of certain plant strains. Some strains of cannabis with a powdery or dusty character are prone to exhibit diamond THC-A crystals as well, resembling powdered sugar. Kief and hash may contain some traces of diamonds as well.
Crystal diamonds within a cannabis extract will appear very small, ranging from snowflake to grain of rice size. Sometimes terpenes also have a crystal-forming structure, and a terpene can bond together with THC-A to form bigger crystals.
Diamonds would be rare to find in the wild
It is only through our selective breeding of cannabis strains that a phenomenon like diamonds would show up in the first place. Wild versions of cannabis typically would not produce the concentrations of THC-A necessary to form diamonds, except as a natural accident. Similarly, hemp, which is bred to have lower THC, doesn’t have the concentration of THC-A where diamonds would form. Thus they are a relatively recent appearance in cannabis history.
Diamonds may also form as an effect of the extraction method. Most live resins or solvent extractions such as BHO are prone to form diamond crystals. In solvent-based extracts, the method and rate of purging the solvent affects diamond formation. It is even possible to produce pure diamonds, with all traces of liquid removed, which is then sold as another kind of extract.
See this production video at YouTube (age-restricted) to view the process in action.