Maybe they thought it had witchy intentions on it, since it was a gift BUT I still believe Christians should believe God overcomes anything they feel is "wrong" in that stone. They could have silently prayed him to cleanse it if they had any doubts and I believe there are examples in the Bible where we are given the okay to do certain things that may not go with our religious practices (using the word religious on purpose here) as to not offend another person, such as eating things we normally aren't to eat. Well, this was an example in the OT, we really don't have that law under Christ but the example was set to not offend, especially when someone is doing something with good intentions, like feeding you (or giving you a quartz out of concern/love).