Question: what are rubies used for in breath of the wild?
Answer: Breath of the Wild Link can sell Rubies for 210 Rupees in most Shops. Rubies are required to purchase the Ruby Circlet from "Starlight Memories" Shop. The description of the Snowquill Headdress reveals it is adorn with a Ruby. Rubies are also used by Great Fairies to upgrade certain pieces of Armor.