Step-by-step: Rendering a stone carving in Photoshop
Start by planning out the pattern of your carving in black and white. It’s much easier to get all your design done in this stage, as changes are much harder to make later on. Add a layer of flat local colour and begin to build up some basic shape and values. In daylight the bottom of your carving will be darker overall. Think about bouncing light, occlusion shadows and how the different interlocking shapes of your carving interact with each other. I recommend restricting your palette to five shades, then make some swatches of colour for your stone and start to render out your carving, trying not to push your values too far. If you want to add a photo texture, use one without strong directional light. Once you’ve placed the texture, set the layer mode to Darken (modes that preserve the original image’s opacity are best for photo textures), create a masking layer and use a toothy brush to build up the mask where the light hits your carving.