Perhaps an even cooler form of crystal would be a four-dimensional one. "How to incorporate this into a 3-d gameworld?" you ask. Simple: all that we mortals can see is a three-dimensional projection of the gem, which may alter subtly as it rotates in four dimensions. This could give these gems a hard-to-hold, ephemeral quality: it would be possible if you gripped them wrongly that they would just fire off into the fourth dimension.
The reason I thought of this was your enumeration of the Platonic solids: in four dimensions there is an extra regular polytope which has no analogue in any other dimension (in 5+ dimensions there are only three Platonic "solids"). Somehow the mysteries of four dimensions seem suited to the properties of a magic-focussing device.
My magical system is based off of a ew chapters in one of those books. The spell crystal idea was a continuation of that.
I actually spoke with Midkemian Press for permission to use and adapt that system. They didn't want it as it only showed up in two chapters of a side plot in a book. Something they actually forgot about. The person I corresponded with said he had to go read the book in question to remember what I was talking about.
In the basics it is similar but I have adapted it and altered it far enough to make it different by far. Part of the reason they didn't remember it. So yes you found me out. But I have permissions to use. Go to Comment
I always liked this one. I used to represent this in game by getting dice in the appropriate size and when someone found one or used one the dice would exchange hands. I would then have them keep them in their pocket or a small dice bag to give the idea of carrying them and the amount of space they take up, that while they are small, they do take up space in numbers.
You come upon a ruined building in the back section of a city park (or other out of the way area of the city). The ruins are fairly overgrown. All that is really standing is a doorway and its frame. If you pass through the opened door, you travel to a different world. If the door closes, there may not be a doorway back to get you back.