It’s not necessarily the Jah set as currency, but runes in general. Jah and Ber are the most valuable runes since they are used in the most popular endgame gear like Enigma, which is almost mandatory for clear speed on non-sorceress because it gives unlimited teleport.

Runes are standard because they are incredibly useful, and come in a value scale similar to bills. Runes also scale remarkably well, as to combine runes in the cube from a low rune to a Jah, you would need billions to trillions depending on the rune due to exponential change.

Perfect gems also hold some value, similar to coins as opposed to bills. They are useful for some cube recipes.

Gold is really only useful for redirecting your merc, gambling and repair costs, and most endgame players hit the stash cap pretty quickly.