Why can't colour be used to identify minerals?

Answer 1

It can be a helpful start, but color is a tricky thing in mineralogy, since small impurities can change the color of a mineral.

I always say it like this: when you think of macaroni and cheese, you always think of it being yellow. A nice bowl of yellow macaroni and cheese (mmm!). Do you use color to identify it? Sure, but you use other things, too: smell, taste, composition (it's noodles and cheese), and such.

But how about if you dropped a bit of blue food coloring in the bowl? Suddenly you have blue macaroni and cheese. It looks weird, sure, but does it smell different? no. Does it taste different? No. Is the composition different? Not really (a small drop of food coloring is added, but what is a drop compared to a whole bowl?). All in all, it's still just a bowl of macaroni and cheese. But if you were just using color, you would have said it wasn't, because mac and cheese "is usually yellow". That "usually" is the kicker which can give a geologist a head start, but it's not an absolute. The drop of food coloring is like little chemical impurities or additions in minerals which can give them another color without really changing their composition or chemistry (this is why quartz has so many color varieties [citrine, amethys, rose, smoky] but at the end of the day they're all just quartz, SiO2)