I only found the game repetitive towards the end game. The game continuously introduce new monsters to hunt and they all require different tactics to tackle them.
I could be wrong (haven't been paying much attention) but IIRC gems only drop off much tougher monsters. It's a random drop so you won't know which gem you get - each gem has different affect, you basically use it by putting them in your equipment gem slots.
?? I have no knowledge on that.. maybe ask @SirJames; ?