Infinite Guitars

Infinite shards take the appearance of a turquoise diamond with a morphing gold center, and eventually are used to upgrade you/your party's guitars. You can actually spot your first obtainable shard immediately after you land on Mikati and get your hovering robot companion. It's behind a locked door below you, and then to the left on a raised, asphalt road, behind some railing. You'll be able to grab it near the end of the level.
Once you've basically completed the level and you're on the other side of the door (by the green sword), head left toward the risen street, and then walk up the most crumbly of that road to continue left. I'm a little embarrassed to admit that I didn't know I could walk up the crumbly stairs and gave up trying to get to the shard after a couple minutes.

Anyways, even if you miss it like I did initially, you'll have plenty of opportunities to get Infinite Shards throughout the next chapter and onward. Cheers.