THC doesn’t crystalize under the same conditions that THCA does.
So if you’ve got THC in your crystal lattice, it’s a contaminant.
Recrystallization done with very careful control of your crystalization parameters should allow you to exclude the undesirables and then wash away any that stick to the outside.
First, pull out your solubility curve for the solvent and solute for the compound you’re working with. Don’t have one? Make one. Or pay someone to do so, or to provide you with theirs.
Next, do a long and slow temperature swing crystalization, staying within the metastable zone once first nucleation is observed.
You should get quite high purity crystals at the end of it, likely very fine like sand.
Finally, once you’ve excluded the undesirables, if you want big rocks, you can then re-crystalize the pure crystals again using your standard parameters to get your desired final physical form.