Geology is utterly not my field although I’m fascinated to read little tidbits here and there, and much appreciate the sense of comprehendable structure your posts add to my aimless stamp collecting.
My reaction to the above was pure Shelley: “Look on my works, ye Newbie, and despair!'” 
To be fair, a range of 20% to 60% quartz is quite a bit of variation in composition.
Speaking of minerals that can’t be reproduced in the lab because of cooling time, my favorite is Widmenstatten (possible sp.) patterns. Acid etch an iron meteorite and you will expose a pattern of two types of interlocking crystals, which differ in percentage of nickel mixed in. This pattern forms only when molten iron is allowed to cool over a period of tens of thousands to millions of years.
Given the way granite crystallises, all it effectively means is how porphorytic the granite ends up looking. Yes, there’s a big difference on paper between a 20% vs a 60% granite, but we don’t really look at the quartz first when we study granite, it’s usually all about that feldspar. I’d lose one mark on a test if I called a syenite a granite, but I’d lose 5 if I called tonalite an alkali-feldspar granite…
Oh, and as a bonus for the non-pro rockheads who may not know about it, I’ll leave you with IMO the coolest granite there is: Graphic granite.
Wow, if I found that I’d be calling NASA to get help deciphering the alien language.
Definitely cool looking. More images.
Mr Dibble may well be able to explain something which has always mildly puzzled me, although hitherto not sufficiently to actually make any inquiries about it.
Why is it that the highest quartz content granite is pink?
Not that I can answer any of the technical questions, but I did just spend some time amongst a great area to study rocks with the composition of granite. I was at Mammoth Lakes, CA, which is located near the crest of the Sierra Nevada. The Sierra is formed primarily of giant plutons of granite that were exposed when the whole block of the Sierra was tilted up on the east side along a series of faults. But also in the area are several rhyolite domes related to the Mono Craters, a result of volcanic activity in the last 50,000 years. The geology of the area is extremely interesting; it sparked a mini interest in the subject in me 50 years ago, an itch I never fully scratched (mostly because rocks weren’t as cool as dinosaurs, or early humans).
Potassium feldspar is usually what makes pink granite pink, but it’s not been my experience that it’s in any way related to the quartz content - I’ve encountered both high- and low-quartz pink granites, and even pink syenites.
OK, one more: Second-coolest granite, IMO: orbicular granite.
I defer to your expertise … I was prompted to ask because the Starrett company (among others) manufactures surface plates in both black and pink granite, and the pink ones are invariably more expensive, because they allege that the higher quartz content of the pink granite leads to less wear, consequently less maintenance is required.
It is quite possible, I suppose, that they have some way of differentiating between high-quartz pink granite and low-quartz pink granite.
I would assume that high-quartz pink granite always has a higher quartz content than black granite ever does? Leastways that is what surface plate manufacturers have believed for over a hundred years … or at least what they have led their customers to believe.
They’re sorta telling the truth about it being more wear-resistant while peddling pure bullshit in their naming. There’s no such thing as “black granite” in geology, only in the nebulous world of rock merchants.
The pink granite plates they sell (which does look like a pink granitoid of some type from what I can tell) *would *have a higher quartz content than their “black granite”, that much is true. This would be because the black plates are not *any *sort of granite - they are dolerite or similar, and as such, wouldn’t have *any *quartz.
Thing to note is, most quartz in granite is clear-to-grey. It stands to reason, the more quartz there is, relative to the pink alkali-feldspar, the *less *pink the rock would be.
