Just looked at a few rock strata in game, and I found interesting results.
For example, I found three layers of granite within the same embark. You can compare these results (Frequent mineral generation). Two of these layers (layer two and three) are actually both bedrock layers over two biomes.
Granite layer one:
Gems: Zircon (red, black & brown), Milk quartz, Prase, Amethyst, Wood Opal, Turquoise, Honey Yellow Beryl
Metals: Tetrhedrite, Bismuthinite
Minerals: Cobaltite, Pyrolucite
Total: 10 Gem, 2 Metal, 2 Mineral, 14 total.
Granite layer two:
Gems: Goshenite, Amethyst, Golden Beryl, Red Tourmaline, Black Zircon, Prase
Metals: Native gold, Native silver, Bismuthinite
Minerals: Pyrolucite, Orthoclase
Total: 6 Gem, 3 Metal, 2 Mineral, 11 Total.
Granite layer three:
Gems: Cat's Eye, Heliodor, Blue Garnet, Turquoise, Smokey Quartz, Clear Tourmaline, Morganite, Aquamarine
Metals: Bismuthinite
Minerals: Cryolite, Pitchblende
Total: 8 Gem, 1 Metal, 2 Mineral, 11 Total.
Matching minerals:
Layer 1 & 2: Bismuthinite, Black Zircon, Amethyst, Pyrolucite
Layer 2 & 3: Bismuthinite
Layer 1 & 3: Bismuthinite, Turquoise
I think that the number of matching minerals is low enough to say that there is no relation between rock layers, same type or not. This creates scarcity and differences is resources even within each layer, regardless of the layer itself. As for global data, each rock layer had an average of 4-5 gems, 1 mineral (like cobaltite or Orthoclase) and two thirds a metal. This makes these granite layers richer than the rest of the layers: This suggests there could be some weighting agasnt or for certain rocks, OR I need more samples.