A cosmic rune obtained in the Runespan is created by siphoning cosmic esshounds (with 27 Runecrafting) and nebulas (with 40 Runecrafting), requiring one rune essence and giving 26.5 and 85.5 experience respectively.
Cosmic runes are worth 0.5 Runespan points and can be used to cross comet platforms. They can also be used to chip astral esshounds. They will disappear and be converted into points if you leave the Runespan.
If you try to bank any runes or essence from Runespan you get the message "A magical force prevents you from banking this item."