I’ve heard that some crystals contain poisonous substances – are these crystals harmful?

It is true that some crystals contain trace elements of poisonous substances, like arsenic. Handling such crystals briefly will not cause harm. It is sensible to follow the usual safety precautions, including keeping all crystals away from children who are still putting all found items in their mouths. Because crystals are cleaned with acids, and some crystals do contain poisonous substances, it is preferable to avoid oral contact with all crystals.