A Unity based AR app to generate a sphere over the camera detected gravel image https://github.com/shubhanksargam/Image-Tracker/blob/main/1200px-Gravel_on_a_beach_in_Thirasia%2C_Santorini%2C_Greece.jpg.
Download the .apk file on your android device, install it and point the camera towards the gravel image above to get the sphere.