Definitely start with the more basic specimens first as they are easy to obtain and generally low cost. You can get a good feel for the range of crystal grades without risking overspending. Plus they're usually available in a variety of types, so you can pick whether you want tumbled, raw, carved, etc.
The developers actually added a pretty great feature rrecently which will help... If you go in to the app and take a look at the Collectors Series, then check out Series 2 - Classics Collection. That list is pretty much perfect for anyone starting out and those would be the ones I'd look at getting first. With $50 you should be able complete that series quite comfortably in tumble and maybe also if buying smaller raw specimens too.