Magic Stone Ornament is an A-grade Ingredient. It currently (as of Season 5) does not contribute to any Treasure Evolutions, so its only recommended use is to extract Magic Powder from it. Alternatively, you can also sell this ingredient for 2,000 coins.
As you can see, the Magic Stone Ornament is pink, but in the Kakao version of Cookie Run it is purple. You cannot buy this ingredient as it is currently not used to evolve any treasure, as mentioned above.