fffffgggg54 said:
Not sure about sapphire, even though sapphire typically means blue corundum, other colors of corundum are also called sapphire. A couple posts would be better tagged pink, aqua, or purple.
Of a sample of 400 posts under the sapphire_(gemstone) tag, I found just over 7% were better labelled a different color than blue, with the majority of those posts being an aqua color. Although many of those posts were by the same few uploaders, which might suggest that they were either unaware of the turquoise_(gemstone) tag or from their perspective it was blue color.
As a comparison, looking at a sample of 200 posts from the blue hair tag. I found that over 17% of those posts would probably be better labelled as a different color, most of those posts leaning toward a purple color. Given the subjectivity of color determination by people, it'll not be avoidable having similar colors be tagged as each other though. Some for example tag hair color based on canon color tagging, even if the character has a color that is closer to another color or the depicted lighting has changed the color their hair in the image.
As for the composition of the gemstone, there is no way to tell the actual composition of them unless someone states it, but that is either something not particularly necessarily being tagged or could be resolved with something like a corundum_(mineral) tag if it ever was actually needed.