In the realm of crystal healing and metaphysical beliefs, there are no crystals that are universally considered “should not be combined” with amethyst.
However, some practitioners might have personal preferences or intuitive feelings about certain combinations. For example, some might feel that a very high-energy crystal like black tourmaline might overwhelm the more delicate energy of amethyst, but this is highly subjective and not based on any scientific evidence.
In general, when it comes to crystal combinations, it often comes down to personal intention and experimentation. As there is currently no scientific basis for specific incompatibilities between crystals, it’s up to the individual to decide which combinations feel right for them.