I am 4 blue essence off of getting mastery level 6 on sona Riot what is this

Buy and sell a champion shard for blue essence, my friend.

I don't really care to display mastery level 6 or higher on any champ, but if I did, I'd just use the champion shards since they always give less than 500 essence which is how much you'd need to pay to turn the tokens into the next mastery level. And I get champion shards for champions I've earned tokens for more often than not, sadly.