Scarlet quartz is like ores that you can break and collect to reduce the rate of sheer cold on your character. Alternatively, you can also use it to deal Pyro damage to enemies or an entity like an ice shard to melt it to reveal items.  As for the Crimson agate, though it looks similar to the Scarlet Quartz, it is used as an offering item to the frost-bearing tree, and you can reap the rewards in return. There are a decent amount of rewards that you can obtain after collecting Crimson agates and leveling up the frost-bearing tree. That's why I highly recommend you to level up your frost-bearing tree for good rewards and invest resources in your characters.