"Rock crystal" is not a term I would ever use as a geologist and I doubt any other geologist (at least in the States) would know what exactly I was referring to if I used it. I would use, "quartz crystals." We tend to use terms that are exact/descriptive rather than ambiguous.
A rock can also be composed of clay particles as well as organic matter: shale and coal, respectively.
As far as solid solutions go, we have specific mineral names for the various phases of a solid solution series. For example, using your olivine example, forsterite is the Mg-rich end-member and fayalite the Fe-rich end-member of the olivine solid solution series. Both minerals are olivines, but geologists like to distinguish between them for a variety of reasons. Namely, compositions of minerals tell us what sorts of conditions exististed when they formed. An abundance of Mg-rich minerals in igneous intrusions usually suggests that the melt was hotter and generally more primitive (i.e., less evolved/differentiated) than one that contains an abundance of Fe-rich minerals (i.e., more evolved/differentiated).
I've not heard or read anything regarding organic material being classified as a mineraloid, but that can change or has changed(?). Clays, such as illite, chlorite, etc. are minerals, but "clay" is also a particle size. So clay minerals, as well as clay-sized particles (of other minerals), comprise shale. (Just an FYI for the readers. )
Iron, the metal, may have a higher melting point than magnesium metal, but Mg-rich forsterite is a far more stable mineral under higher temps and pressures (as present in the mantle) than the Fe-rich fayalite. Therefore, forsterite will crystallize first from higher temp ultramafic/mafic melts, rather than fayalite.
Ah yes, I see the inclusion of organic material. Hmmmm... not sure I agree, but not really worth arguing it. As for the clays, I was simply elaborating a bit for the readers. Not everyone knows clays can also be minerals.
Geology can be pretty "touchy feely," which is what drives the engineers crazy. Occasionally, there just isn't enough evidence to be sure, as you very well know, so we have to go with our gut. The longer you've been a geologist and the more rocks you've seen, the better your chances of being closer to the truth.
I'm modeling an ore deposit right now and I can tell you I'm digitizing a lot of touchy feely lines at the moment.