I don’t believe nor feel there are any bad combinations of crystals out there that cannot be put together. However, I do believe that some crystals can cancel each other energies; not in a bad way, but more like sending a confusing message since they have opposite vibrational frequencies.
For example, I would not suggest my customers to combine Amazonite with Carnelian. Since Amazonite is a calming stone while Carnelian is a high vibration crystal.
First of all, you’ve got to ask yourself what are you trying to achieve here? Are you trying to be calmer or to have surge of life force ?
On another possibility of bad combination is when one is wearing too much at the same time, like layers of malas or crystal necklaces. Usually when you do this, your body will know. Talking from a personal experience, one time I made a full mala using Hematite and Black Onyx with Black Pearl pendant in addition to my Herkirmer diamond choker that I wear at all time (one of Herkirmer strengths is that it amplifies the vibrations of other crystals) and soon after I was feeling heavy, sluggish with twinge of light headedness. And as soon as I took off the mala, all the discomfort dissipates. That means it was a bad crystal combinations for me, but then, it might works well for someone else that actually needing all those super grounding energies.
I’m not saying you can’t wear powerful crystal combinations but less might works better. Again, if you listen to your body or pay attention, they will let you know if you are wearing or using a good combinations or not.