I have been messing around with this setting, and so far I am finding that a setting of 100 gives WAAAAAAY more variety of minerals than one found in .18, to the tune of a minimum of 3 vein types in every single layer, some layers even have 4 or 5. I have also found that changing the setting and regenning with all the rest of the settings the same leaves the geology of the world the same, but the biomes are a little different, as is the evil and savageness. For example, I generated a world with the setting at 0 (manually editied the file to go that low) in order to see if there is an appreciable difference from 100 (there is a minor increase in quantity and variation), but what really threw me was that the site I was checking was untamed wilds with a setting 0, and terrifying with 100. A few further gens with only changing the minerals setting (with as minor a change as 1 a couple times just to see) gave me results of mithful savage and haunted as well. This could be useful to know, if one had a site they like, but say wanted it to be haunted instead of mirthful, you can try changing the minerals setting in minor increments, regenning every time, until you get the biome you like. I am going to do some further experimentation to see if there is any rhyme or reason to it.