Kyanite
kyanos — the deep blue
Kyanite is mineralogy’s lesson that direction matters: scratch along the blade and it yields like glass at 4.5; scratch across and it resists like quartz at 7. The French still call it disthène — “twice strong.” The blue can stand beside sapphire, for a fraction of the ceremony.
“One stone, two hardnesses — the mineral that taught direction matters.”Mineralogists’ lore · disthène, “twice-strong”