calcium sand is actually the worst thing to put them on, yes they may be from the desert but the desert that they come from is mostly clay and slate rocks. that wood is big enough that *if* it gets into the gecko's mouth, the gecko will be able to spit it out, it does not have that advantage w/ sand and the sand will get clumped up in the digestive tract causing impaction, in a gecko that size there is a much lesser risk of sand impaction but that risk is still there and is increased on calcium sand.