Luminescent Spine is an item that you get when you catch a firefly. These things are found in the wild, and can be spotted at night. They can be somewhat difficult to catch since they are quite elusive and easy to miss.
The item is used to make a portable waypoint. Even though you can only have 1 active at a time, these things can be pretty convenient to use, so you can consider farming for the fireflies if you want to make a lot of them.
Fireflies can (only?) be spotted at night time, or at least this will make them a lot easier to see. Be sure that you set your in-game time to around 6 PM before going to these locations.
You can change your in-game time with the "time" function, available on the left side of the screen, below the mailbox icon.
When you see them out in the wild, they look like sparkling yellow dots. Can be easily missed if you aren't paying attention. When you get close to it, you only have a short moment to catch it, otherwise they will fly away to a different area close by, but they can also just disappear entirely, so try to make it count.
You may not be able to find them right away when you reach certain areas. You can try to wait there for a few minutes and see if they show up, or just skip the area entirely since they might not be there for you at the time.
You can also sometimes find them by destroying bushes. Not sure if it works with all bushes, or only in certain areas, but the odds are not that big.
Guyun Stone Forest
The easiest place to start getting these fireflies is most likely in Guyun Stone Forest, in front of Domain of Guyun door. You may not see them right away, but you can stand there for a few minutes and they will slowly start to appear.
Southwest Liyue Harbor
You can get some fireflies very close to the teleport waypoint, and a little further down south on a tree as shown in the picture above.
Several Fireflies can be obtained in Lingju Pass area shown above. It's in the part with 3 abyss mage and some hilichurl archers.
Spotted in this location in Jueyun Karst, inside a "tunnel" like area. The fireflies were close to the wood pieces.
About 3 were found in this area in Cuijue Slope. The fireflies are close to the dead trees shown above.
In Tianqiu Valley, some were found close to the water area in the marked area above. Also was able to find a firefly after destroying a bush in the area.
Some were found here, in the grassy area southeast of the Statue of the Seven in Dihua Marsh, somewhat close to the river.