Deffo leave 5 stars as this will help them out big time on the store! Also use the in-app reporting feature if you see any errors/typos etc. Might be worth noting that if you intend to rate lower than 5, this won't actually be helping them as it will just serve to drop their average rating, so maybe abstain from rating if you dont want to give a 5 for some reason.
From my email exchange with them, I am pretty certain they work with geologists to produce the content, but maybe there's something else we can do. It's easily the best crystal app and community and for good reason.