A sol rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 amn runes and a chipped amethyst together to get a sol rune. You can also obtain a sol rune from nightmare countess. You can get a sol rune from the nightmare hellforge quest.
Sol runes are highly sought after at the start of the game, because they are used in useful low level runewords like Insight and Lore. The easiest way to obtain a sol rune, is to farm the Countess in Nightmare difficulty.
A sol rune requires level 27 to use.
Horadric cube recipes with sol runes
transmute 3 sol runes and a chipped sapphire to create a shael rune
transmute normal unique weapon, a sol rune, a ral rune, and a perfect emerald to create an exceptional unique weapon of the same type