An amn rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 thul runes and a chipped topaz together to get an amn rune. You can also obtain an amn rune from nightmare countess.
An amn rune requires level 25 to use.
Horadric cube recipes with amn runes
transmute 3 amn runes and a chipped amethyst to create a sol rune
transmute normal weapon, a amn rune, an ral rune, and a perfect amethyst to create a socketed weapon of the same type
transmute normal shield, a amn rune, an tal rune, and a perfect ruby to create a socketed shield of the same type
transmute normal rare weapon, an amn rune, an ort rune, and a perfect sapphire to create an exceptional rare weapon of the same type