Compare two stones
Why compare two stones
The question « amethyst or charoite? » has no answer in the abstract: it has one the moment you set them side by side. Seven on Mohs against five, transparent against opaque, two euros a carat against thirty: the table settles in one line what a paragraph takes ten sentences to explain.
The three rows that matter most before buying are hardness, which says whether the stone will survive an everyday ring, care, which says whether it tolerates water and sunlight, and the indicative price, which sets the order of magnitude. The rest settles confusions.
Two stones that look alike are not worth the same
Most flea-market disappointments come from well-known pairs: jadeite and serpentine, ruby and spinel, imperial topaz and citrine, turquoise and dyed howlite. To the naked eye they merge; on hardness, density and refractive index they part at once.
That is what the table is for: it does not say which one is « better », it says where they diverge, and therefore which test to start with.