I was thinking about it, and we don't really have any stone/rock/dirt-related pets - or really even sand. The pets don't need to necessarily be made out of the materials, but it could be applied to them, kind of like how glade pets have flowers, mushrooms, and butterflies incorporated into their fur.

Sand pets could have parts of their fur fading out into whisps of sand like how reborn pets have fire. They could also have some other small features onto them - maybe thick lines around their eyes for the desert environment, and particular markings (maybe stripes?) throughout the theme.

Stone pets could either be made out of stone - like gargoyles, but maybe a different color scheme (rather than just slate grey, maybe some warm reddish browns, otherwise the magnus would be pretty boring), or they could have rock incorporated onto them, kind of like armor or a protective shell.