I was asked a very good question which had me doing a bit of research this morning that took longer than expected but had me really "putting on the thinking cap". The question was regarding gemstones and the Seals of Solomon - which stones for each of the Pentacles?
From what I found after searching through the Keys of Solomon books and then looking online and couldn't find anything either (and I'm certainly not saying this definitively), there was nothing on the subject. However, after reading through quite a few sites and looking through my own books, I came to the realization that the use of the Pentacles and their associated colors can easily determine the stones to be used with them in ritual.
Fire & Air stones are clear or translucent and easy to see through, while the Earth & Water stones are opaque and cannot be seen through. When considering the Pentacle to be used and the color associated with it, you may also want to consider the Element of that Pentacle's planetary alliance and choose a stone accordingly:
Earth (opaque stones): Saturn
Air (clear stones): Jupiter & Mercury
Fire (clear stones): Sun & Mars, also Jupiter
Water (opaque stones): Moon & Venus, also Saturn