They messed up this part in the remake. In the original the right crystal made a sound like a breakable wall when you hit it. The right one is a solitary crystal in a screen where there aren't any enemies.
Anyway, I've just finished the game. Was lvl 56 at the end and it only took roughly 8 hours to beat. It's kind of funny that, for today's standards, it is a rather short game. When I originally played it more than 20 years ago (fuck, I'm old...) I thought it was absolutely massive and epic. Well, back then I was stuck for weeks at that infamous oasis puzzle and only managed to get past it when I came across the solution by coincidence in a gaming magazine. I think that puzzle managed to piss everybody off at that time. "Palm trees and eight"? Fuck you!
Still, someone who has never played it before should get a good 10-15 hours out of it.