The Lumitoile is a Fontaine local specialty and a brightly-colored starfish that can be found by climbing walls and buildings in the northern parts of the region. I highly recommend harvesting it whenever you encounter it in the wild, as there aren't enough to fully Ascend a character in one day.
Lumitoile lives both above ground and underwater and is easy to spot due to its large size and bright appearance. It's used as an Ascension material for Neuvillette and possibly other Fontainian characters in the future, so be sure to grab as much as you can.
Where to farm Lumitoile in Genshin Impact
Lumitoile is a large, squishy-looking starfish. Near the shorelines, you'll usually find it on the sand, bathing under the sun. However, in underwater areas and places away from the shore, this giant starfish is usually stuck on walls and buildings, just like Philanemo Mushrooms. So when you start searching for them, keep your eyes peeled as you might need to climb or swim to higher parts of buildings to find the coveted Lumitoile.
If you run out of these starfishes in your World, I recommend entering a friend's World through Co-op mode and asking them whether you can farm in theirs.
Reference the image above for all the Lumitoiles you can find above ground. They're mostly clustered in the shorelines of the Liffey Region, so they're easy to spot and the farming routes are easy to remember.
Reference the image above for all the Lumitoiles you can find in the Liffey Region's underwater area. The Lumitoiles in the underwater "land" areas are located underground and are only accessible while you're submerged, so you can get to them via the same route.
To find more local specialties in Fontaine, check out our guide on All Romaritime Flower Locations in Genshin Impact here on Pro Game Guides.