When we first suspected Bethany had some form of autism, I put her on a gluten-free, casein free diet. Gluten is a protein found in wheat (all forms, including spelt), barley, and rye (try telling this Swedish girl she can't have Limpa bread----WAAAAAAAAAAAHHHHHHHHHH!!!!). Casein is a protein found in milk (unlike lactose, which is the milk sugar). That was not long after I started my Pampered Chef business. Later, we had her tested for food sensitivities & found out she is moderately sensitive to gluten, but does fine with casein (and lactose for that matter). Then along came my surprising diagnosis of Celiac Disease which meant NO gluten. Not just "don't eat gluten for a while, then you can add it in." It means NO GLUTEN EVER. Why not? Well, besides the unpleasant aromas it produces when my stomach can't digest it (ahem), untreated Celiac Disease (ie eating gluten when you have Celiac disease) WILL, not might, but WILL lead to lymphoma. That was enough to scare me out of cheating on the diet. No, a crumb will not send me into the chemotherapy clinic, but it's just easier to avoid it totally. Some people DO react to as small as a crumb of gluten.
As a Pampered Chef consultant who owned every baking stone PC made, my question was, "Can I still use my baking stones and be gluten free?" A friend just asked me this question, so I am going to tell you what PC told me, then I will tell you what I think. PC's response to this question was that it is HIGHLY UNLIKELY that any gluten would remain on a stone once it has been cleaned. However, if you own a baking stone, you know that they build up a coating on them the way that an iron skillet does. It's called "seasoning." You also know that PC says not to use soap on stones for 2 reasons: 1) It breaks down the seasoning on the stone and 2) your stone will absorb the soap and food will taste like soap. For the record, I have put soap on my stones and have not felt like I was having my mouth washed out with soap when I ate, but I do not use a lot of soap. I normally only do this if the stone is super greasy and even baking soda scrubs don't get rid of the grease. So, back to gluten. Because Pampered Chef cannot be 100% sure that all gluten is removed, they do not recommend cooking/baking with the same stone for gluten-free cooking that you used for gluten cooking.
For the person who owns a "pizza stone" and maybe one other stone, this isn't a big deal. Order a new large round stone, or host a party & get some stones cheaper. For the person who owns every stone, and sells Pampered Chef to get free products because I can't buy them full price (though they are worth every penny, just don't have that in our budget), this just isn't a reality. So, I do use the same stones for both, but I am sure to thoroughly scrub my stones if I've baked gluten products on them.
The advice I give customers is to either purchase stones to keep "safe" (gluten-free, no cross-contamination), or use parchment paper on your stones if you want to be SURE there is no gluten. If you are highly sensitive to gluten, this really is the way to be 100% sure you are not getting any bits of gluten. However, if you feel comfortable with the way you clean the stones and you are not highly sensitive, then you can try using the stones you already own. It is the oil that stays behind to cause the stones to season, not grains. I would tell someone with a peanut allergy NOT to use the same stone if peanuts or peanut oil had been on the stone because that could transfer the allergen (though I think Alton Brown's theory is that peanut oil does not cause an allergy in peanut allergic people).
There is your very long non-conclusive answer, but I hope it gives you some guidance in using your stones! I still say stones are the best way to bake!