Relics and Runes


Could someone help me understand the runes that can be inserted into relics?
At a high level, it's crystal clear that they add benefits to a given relic, which is great, but I can't seem to find runes in my inventory.
I assume runes are not a limitless resource, otherwise the abilities added by runes would just be standard. So with the assumption that I don't have a bottomless pit of runes, I want to make sure I'm using them under the best circumstances, but for the life of me, I can't find runes anywhere in my inventory.
Hoping someone can educate me.
Thanks in advance.
3
Comments
Thanks @Feuerschwer , @CoatHang3r , and @Buttehrs for getting me sorted.