(Posted this elsewhere, as a response to another topic - though it might be worth its own topic.)
I'm starting to believe that, on a game map (or perhaps a world "Region"), area blocks aren't actually, purely random as to what ore and gems they have. Instead, nearby area blocks tend to mirror each other in terms of what ores and gems they have, or which are predominantly found.
So, let's say your map is 6x6, and has 5 layers of granite across the whole - 180 area blocks worth of granite layers. By the raw's, you might expect to find all of the following, at least somewhere on your map:
* Native gold
* Native silver
* Horn silver
* Galena
* Cassiterite
* Tetrahedrite
* Bismuthinite
Plus a full spectrum of gems.
But that's rarely (never?) the case.
What seems to happen, is one ore or gem is dominant, and others lacking entirely. If the map has cassiterite in more than one or two blocks, then get ready for lots of cassiterite, more than you'd ever want - and not as much of anything else. On the other hand, if you've explored 20 blocks and found no tetrahedrite, then suck it up, this area just doesn't have tetrahedrite, or not very effin much, regardless of what you might expect from the stone layers.
Too many maps I've seen "GOLD!" and then on maps with the same layers... bupkiss, across 6x6 and many layers thick. And if you find moss agate or pineapple opal or whatever, get ready for more - lots more, even if any/every other opal or agate sub-type "should" be there. I think the game mechanism grabs a "seed", one favored ore and/or gem (or a few), and weights those over others for the area during world gen. It's why some maps have rock crystals everywhere, and some have none. Why some maps have no copper or iron, or coal, or bauxite, even when you'd expect them to (or hope). How civilizations differ, instead of being generic from an inevitable bell curve of minerals present in every block.
Not 100% sure of this theory, but it's growing the more maps I see.
Anyone else see this, or think this is an unjustified conclusion? I'm not convinced one way or the other, so speak up either way, just looking for more input.