Now that I've completed it, I can say with confidence this is one of the most well balanced defence games I've played for some time. 5 and added to favourites.
Actually, you can use any gem (pure or not) of any grade to activate a shrine. However, Using a PURE gem of grade 6 or above will result in a Shrine Burst amulet, which gives 300 exp to your score, this bonus can be tripled in modes worth triple exp.