Crystals for concealment

[stories_of_andruste]

What crystal/s would typically be used for concealment magic?

My antagonist has created a poison and wants to make sure that the ingredients/source of the poison cannot be detected at a later point in time. So this crystal would be used in a spell that hides the poison magic.

Obviously it's fantasy but I want it to match the associations/energetic properties in our world. Would appreciate advise from a modern-day witch or crystal hobbyist.