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[Cromage]

Not sure if this is a bug, a counterintuitive world gen quirk not described in the wiki, or something else:

Here's what I did:

1) Started a new world with default levels of mineral scarcity (2500)
2) Started an embark with "deep metals."
3) Ran prospect; noticed that there were three stone layers of igneous intrusive stone starting at Z=137 but only 8 mineral types (3 ore, 5 gem)
4) Ran reveal:
- Granite layer (the bottommost) generate normally with the normal mineral veins
- Diorite and Gabbro layers had no mineral veins at all, whether metal or gem
5) Ran unreveal and "fixveins" which "restored missing references to 573 mineral inclusion tiles" but didn't make any notable changes.

Judging from the wiki, 8 mineral types is extremely low. So the question is: was I just extremely unlucky or is the default setting to have all ore in just one layer? Are there any other settings that might have made a difference?

[Fleeting Frames]

I don't generate worlds with that mineral scarcity, but the ore part is what I'd expect with "Deep Metals", especially as Diorite is noted on the wiki for its' layers' uniform colour. Newbies are suggested to embark in areas with shallow and deep metals both for a reason, after all.

Higher scarcity definitely means less gems, though I haven't paid attention to the typical numbers of clusters in a 48x48 like I have with veins.

[PatrikLundell]

Different layer minerals support different veins and clusters, but I haven't looked at what supports what: I just take what I get.

It should also be noted that DF's site report doesn't take SMR cutoff into consideration, so it will report stuff that would have appeared had the SMR been further down (and similarly, there's a soil erosion process that can cause expected clay and aquifers to fail to appear).

[Cromage]

I don't generate worlds with that mineral scarcity, but the ore part is what I'd expect with "Deep Metals", especially as Diorite is noted on the wiki for its' layers' uniform colour. Newbies are suggested to embark in areas with shallow and deep metals both for a reason, after all.

Higher scarcity definitely means less gems, though I haven't paid attention to the typical numbers of clusters in a 48x48 like I have with veins.

Possibly, but that'd also be inconsistent with the wiki and/or reports from earlier versions (possibly due to version change?). The wiki entry here suggests that Shallow Metals are designated for the top 2 stone layers for most biomes, where in this save metals aren't found until the 6th layer, and the observations in this thread say that they start about 15 levels below ground level (whereas this one requires 30)

Also there are no gems at all in any of those blank layers. That is unusual for Gabbro. If world gen is designed to do this, that's fine for my fort (I see a lot of silver hammers in my dwarfs' futures), but if I didn't use DFHack I would be very frustrated digging several layers through plain stone!

Different layer minerals support different veins and clusters, but I haven't looked at what supports what: I just take what I get.

It should also be noted that DF's site report doesn't take SMR cutoff into consideration, so it will report stuff that would have appeared had the SMR been further down (and similarly, there's a soil erosion process that can cause expected clay and aquifers to fail to appear).

According to the wiki at least, Gabbro is supposed to have a bunch of sublayers as well as being the only source of garnierite.

SMR = semi-molten rock, right? In this save, magma doesn't appear until the bottom portions of the granite layer (which is the only layer that has any veins of any other material), so that's well below a bunch of layers that theoretically should contain minerals.

[PatrikLundell]

Yes, SMR = Semi Molten Rock.
The geo biome typically contains layers defined a lot deeper than the SMR cutoff provides access to (it support a maximum of 16 layers).

The list of what can be present is just that. There's no guarantee any of it will be, although I would expect a low scarcity would make it unusual for nothing to be present, but I don't think the process is understood in any detail. There are two levels to the process: the first one is the generation of the geo biome itself, where some process determines what layers are present and what is/may be in them. Then there's the embark generation process where what's defined in the geo biome gets generated, and there's something which looks like a measure of abundance in each of the vein/cluster/inclusion entries that seem to correlate somewhat with how abundant it ends up being.

[Adequate Swimmer]

Your site simply didn't have a river or lake in pre-history. There was no water to extract the ore and deposit it on the bottom of something else. Notice the almost complete lack of sediments on the map.
Look for a site with flux+shallow metal.
You can make the site serviceable by arming a bunch of guards with wooden swords and shields and simply stealing the metal from your neighbors. Or develop your sheep industry until you have enough finished products to import a large amount of metal.

[Luriant]

Or more exploit.

Magma Forge > Make Steel Coins
Magma Smelter > Smelt Coins
Repeat

Every Coin stack melted give 110% initial ore. After 10 works, you have 1 extra Steel bar.
Split the coins give 500% initial ore, but is a lot of work in the trade menu.

[Fleeting Frames]

If going for exploit, might as well use leggings or spiked balls (both 50% return), or gauntlets/high boots to train armorsmith faster at same time. Coins are necessary for platinum, gold or adamantine.

In regards to shallow vs deep, worth noting iron can be deep.

[Cromage]

Yes, SMR = Semi Molten Rock.
The geo biome typically contains layers defined a lot deeper than the SMR cutoff provides access to (it support a maximum of 16 layers).

The list of what can be present is just that. There's no guarantee any of it will be, although I would expect a low scarcity would make it unusual for nothing to be present, but I don't think the process is understood in any detail. There are two levels to the process: the first one is the generation of the geo biome itself, where some process determines what layers are present and what is/may be in them. Then there's the embark generation process where what's defined in the geo biome gets generated, and there's something which looks like a measure of abundance in each of the vein/cluster/inclusion entries that seem to correlate somewhat with how abundant it ends up being.

Okay, that helps, thanks. It sounds like the geo biome generation decided that some igneous intrusive layers would have no minerals for some reason and that other layers would, and then embark generation just spread the minerals accordingly.

Your site simply didn't have a river or lake in pre-history. There was no water to extract the ore and deposit it on the bottom of something else. Notice the almost complete lack of sediments on the map.

If that were the case I'd expect the uppermost layers to be the scarce ones, and they actually do have mineral clusters (gems and orthoclase mostly)

It's the lower-middle layers that are the unusual ones, which I would expect to be driven less by river deposits and more by volcanic activity.

I may end up just doing some more world generation to figure this out. I didn't know prospect could be used before embarkment and that might help me get a feel for why sites end up the way they do.