The post on an earring is the straight pin or wire integrated into the earring's front element that passes through the piercing. The separate finding at the back, called a back or nut, is what keeps it in place.
Stone Story
At Mohs 9, second only to diamond and the same hardness as sapphire, ruby resists surface abrasion at a level most colored gemstones don't approach. The corundum crystal structure that produces that hardness allows ruby to maintain its polish and surface quality through daily wear in rings, bracelets, and all contact-prone applications without the protective-setting qualifications softer stones require.
Built For Ease
Clutch back placement on the post directly affects how an earring wears. If you position it closer to the lobe, you'll get a flat, snug stud fit. If you leave it a bit more exposed, it creates a small gap that tilts the earring forward and changes how it hangs.