Marble is a rather necessary resource pretty early in My Time at Sandrock. Among other uses, it’s required to upgrade your workbench to a more advanced level. So where can you find it?
Marble is pretty easy to come across, and you can get it from the relatively safe area around the workshop. It’s very easy to overlook, so if you didn’t notice the deposits of it, that’s understandable.
You can get marble by mining hard rock. These appear as pretty ordinary-looking boulders but can be differentiated from gravel by their grey tops. That little bit of grey can stand out pretty well from the yellows and oranges of the desert.
Note that, in order to get this, you need the bronze pickhammer. To get the bronze bars required to create it, you’ll first need to complete the lift and go a few floors down in the ruins. You’ll eventually come across tin ore, which can be combined with copper ore in a furnace to create bronze bars. Once you’ve created a bronze bar for the first time, you can then buy them from Hugo’s store, Hammer Time. It’s also at that store that you’ll find the sharpening stone that is also needed for making the bronze pickhammer.
In any case, once you have the bronze pickhammer in hand, you can then break apart hard rocks and obtain marble.