The terms felsic and mafic refer to the mineral composition of magma. Magma naturally contains plenty of iron and magnesium from the Earth's core, as well as silicon and aluminum. This makes magma mafic (mafic being a term combining magnesium and ferric, the Latin word for iron)
The iron and magnesium, however, makes the magma denser, and can cool off and solidify at lower temperatures than silicon, leaving only the silacious magma still molten, and in that state, it is also less dense. In this state, it becomes felsic magma.
Felsic stones are less dense, lighter-colored, and often harder than their mafic counterparts.
Oceanic volcanos, with their thinner crust to break through, and constant eruptions are more mafic than the inland volcanos, with their rare eruptions and thick crusts. Mafic volcanos are generally less explosive and dangerous because of this.
Also, mafic stones (Gabbro/Basalt) are denser and more common where the crust is thinner - near the ocean. Most continents are primarily made of (felsic) granite at the lower levels. The probability of mafic or felsic igneous stones should therefore be a function of how much elevation there is - a mountain range should almost always be granite at its base unless it is an active island volcano or the result of some folding of a continental plate collision like California.
As I already discussed, ultramafic kimberlite and peridotite should be its own type of layer stone. Ultramafic stone is rare anywhere near the surface, but was common during the very early formation of the Earth's crust, and survives in the deepest portions of the crust, in places where the crust has been thrust upwards to the point where the deep peridotite has been exposed, and in kimberlite pipes. As I said in the kimberlite pipes section, diamonds would ideally only occur in kimberlite pipes.
Because these mafic and felsic stones are chemically different, it should follow that the ores you find in the stones and the way in which the stones erode in worldgen are different. Granite (intrusive felsic) is some of the hardest, most durable stone around, and makes up the base of most of the continents. Basaltic rock (extrusive mafic), meanwhile, is easily eroded down to black sand, and obsidian will functionally slowly dissolve in water.
From
this site, "Mafic rocks are high in iron and magnesium minerals, primarily Olivine, Pyroxene, Amphibole/Hornblende and aluminosilicate minerals like Calcium Feldspar/Labradorite and Calcium/Sodium feldspar mixes. Felsic rocks contain fewer iron magnesium minerals but are high in aluminosilicate Sodium Feldspar/Albite and Potassium Feldspar/Orthoclase, as well as sheet silicates/Biotite and Muscovite Micas and pure silica tetrahedrons/Quartz. Intermediate rocks contain some mafic minerals and some felsic minerals."
Wikipedia, also has a
useful chart in demonstrating where you are more likely to find the orthoclases compared to the olivines.
The most difficult-to-find data, however, regards the most relevant data - which forms of igneous stones (especially the nearly-identical igneous extrusive stones) are more likely to contain what minerals.
This page is a Canadian Geological Survey, performed by the University of Ottawa. It is not an easy read by any means, and I say that being the guy who wrote (and researched) the massive Improved Farming thread.
Straining through the overflow of technical jargon and dry text, I found the following nuggets of useful data:
"As expected, the three deposit types dominated by mafic volcanic and volcaniclastic rocks have the highest Cu grades, whereas the two felsic-dominated deposit types contain the highest Pb and Ag contents. The bimodal felsic deposit group contains the highest average gold. Mafic-ultramaficdominated systems can also contain Se, Co, and Ni."
Mafic means basalt, and this says that they have the most copper (native copper). Selinium is not covered in the game, but Co is cobalt/cobaltite and Ni is Nickle/Garnierite. Felsic means rhyolite, and it has more lead and silver (galena, but also potentially just native silver), as well as gold. Shifting the way in which ore appears in game to favor this skewing would help differentiate the types of stone.
"Mafic-dominated, bimodal mafic, and bimodal felsic host rocks are dominated by effusive volcanic successions and accompanying, large-scale hypabyssal intrusions (Fig. 17). This high-temperature subseafloor environment supported high-temperature (>350°C) hydrothermal systems, from which may have precipitated Cu, Cu-Zn, and Zn-Cu- (Pb) VMS deposits with variable Au and Ag contents. Areally extensive, 1 to 5 m thick, Fe-rich “exhalites” (iron formations) may mark the most prospective VMS horizons (Spry et al., 2000; Peter, 2003) (Fig. 18A). These exhalite deposits consist of a combination of fine volcaniclastic material, chert, and carbonates. They formed during the immature and/or waning stages of regional hydrothermal activity when shallowly circulating seawater stripped Fe, Si, and some base metals at <250°C and precipitated them on the seafloor through extensive, but diffuse, low-temperature hydrothermal venting. Formation of exhalites on a basalt-dominated substrate was commonly accompanied by silicification and/or chloritization of the underlying 200 to 500 m of strata (Fig. 18B). Examples of this are observed in the Noranda, Matagami Lake, and Snow Lake VMS camps (Kalogeropoulos and Scott, 1989; Liaghat and MacLean, 1992; Bailes and Galley, 1999). In felsic volcaniclastic-dominated terranes, the generation of Fe-formation exhalites was accompanied by extensive K-Mg alteration of the felsic substrate, as recorded in the Bergslagen district of Sweden (Lagerblad and Gorbatschev, 1985) and in the Iberian Pyrite Belt (Munha and Kerrich, 1980).
Mafic, felsic, and bimodal siliciclastic volcanic assemblages tend to host volumetrically smaller mafic and/or felsic sill-dyke complexes, and generally contain Zn-Cu-Co and Zn-Pb-Cu-Ag VMS deposits, respectively. More Cu-rich deposits, such as Neves Corvo in the Iberian Pyrite Belt, may also be present in settings proximal to discrete extrusive complexes. The district-scale semiconformable hydrothermal systems consist of low-temperature mineral assemblages, with Mg-K smectite and K-feldspar alteration overlain by extensive units of low-temperature Fe-Si-Mn deposits. Other types of iron formation in VMS districts are interpreted to be products of plume fallout from high-temperature hydrothermal venting, or collection of hypersaline brines within fault-controlled depressions on the seafloor (Peter, 2003). Iron formation horizons can extend for tens of kilometres, as in the Bathurst VMS camp in New Brunswick (Peter and Goodfellow, 1996) (Fig. 18C), the Paleoproterozoic Bergslagen district (Allen et al., 1996b), the Devono- Mississippian Iberian Pyrite Belt in Spain and Portugal (Carvalho et al., 1999), and the Mississippian Finlayson Lake camp, Yukon (Peter, 2003). Mineralogical variations within these regionally extensive iron formations, from oxide through carbonate to sulphide, are indicative of proximity to more focused, higher temperature hydrothermal vent complexes and also reflect stratification of the water column in the basin. The mineralogical variations are accompanied by changes in element ratios such as Fe, Mn, B, P, and Zn (exhalative component) versus Al and Ti (detrital clastic component) (Peter and Goodfellow, 1996)."
Some interesting images are down near the bottom of the page, as well:
http://gsc.nrcan.gc.ca/mindep/synth_dep/vms/images/fig04.jpg This one is especially good, since it shows the type of lava or magma produces that produces specific types of mineral wealth - the bottom right one, for example, shows a basaltic pipe filled with iron ores, among other minerals. Notably, it shows magnetite in a basalt layer.
http://gsc.nrcan.gc.ca/mindep/synth_dep/vms/images/fig22.jpghttp://gsc.nrcan.gc.ca/mindep/synth_dep/vms/images/fig11.jpg