These are some mesh rocks, a part of the #FYIBeachKit system designed to make sure that your beach has matching style and textures.
There are 24 rocks in this kit, 12 unique meshes with two different texture variants each. One for a solid rock, and another that is faded from the bottom so you can blend it with water, sand, etc and it looks a lot more natural sitting in the sand, for example.
Each rock is between 5-13 land impact at the default size, which is generally 8-10 meters on the largest axis.
Please check out the rest of the #FYIBeachKit for more matching mesh beach goodies