What is the ruling on using crystal stones for medical treatment; is it permissible, or is it considered a form of polytheism (shirk)?

There is no harm in seeking treatment and medication with anything proven beneficial and effective, whether religiously or scientifically. The Prophet, peace and blessings be upon him, permitted seeking treatment. Medication is a means to healing. These means can be religious (like the Quran and supplication) or physical (like material medicines known through religious texts or experience). Their effect must be direct, not imaginary. If the benefit of these stones in treating a particular illness is proven by experience, then there is no harm in using them, as long as they are a normal means to combat the illness, coupled with reliance solely on Allah and not on the means itself.