Ok, this has a slight spoiler in it. Even for all of you who've breached the HFS and bathed in the blood of demons, you might want to avoid this information. It's gleaned from the code, observing patterns and finding something that repeats. So if you like exploring for your metals, turn away! Otherwise, you may find the challenge of ore hunting gone :3
So, basically, minerals spawn in a certain way. This way, to my observations, is 1 large cluster, 2 veins, and a good half-dozen small clusters. This means you might have Microcline, Hematite, Cobaltite, and then a half dozen assorted gems, or the odd stones like saltpeter that generate in small clusters. Or it might be Orthoclase, Galena, Native Silver, and a half dozen diamonds! (Not really, diamonds have special spawn patterns). The point is, you get one large grouping of your usually useless stone, although gypsum is a large cluster that can be used for something, and then two veins of something, which can be metal or not, because stones like hematite and cobaltite are both vein stones, and finally some assorted small clusters, the 3-8 little groups of precious gems, usually.
And this repeats per embark tile. So a 2x2 embark will have 4 large clusters, 8 veins, and some 24 clusters. This is, although, at Scarcity 100, but the same thing should repeat in less plentiful circumstances at higher scarcity. The basic point remains that each embark tile is a "unit" of ore, and will have roughly the same kind of pattern. At Scarcity 100, the easiest way to find metal is to find Microcline, remove the microcline, and expose the veins that run through it. There's a lot of random placement, but it's still the same basic design.
Observe a zoomed out 16x16 embark that I managed to run. The full zoom out doesn't show the full 16x16, but you can still see the repeating pattern rather clearly.