This guide will let you know where to find Starconch in Genshin Impact.
Starconch looks like a blue shell with a star on it, and you can find it in Guyun Stone Forest, as shown in the image below.
In these different islands, you have to search around the beach areas of these islands, and you will easily find Starconch laying on the sand near the water, as shown in the image below.
It is a material used for ascension for some heroes like a child, launching in the upcoming update. So all of the locations to find these shells are given below.