Move a slider in the pattern panel and the stone is redrawn on the spot. Scale sets how close you stand to the rock, vein density and vein sharpness decide how the veins come out, and turbulence and warp bend the way they flow. Raising detail adds finer shading, and grain adds the roughness of a cut surface.
The circles in the color panel are kinds of marble. Pressing one swaps the three colors and the vein character of that stone, and you can still pick the base, vein, and shade colors one by one. When you want the same settings with a different figure, press shuffle pattern.
Choose a size in the export panel and press save PNG. What you get is redrawn at that resolution, not the shrunken view on screen, so a 3840 × 2160 wallpaper comes out at full size.
Panels move when you drag their header. They stick to the edge of the screen and to each other, and a panel resting against an edge drops the rounding on that side. Double-click a panel title to switch to the fixed layout, which keeps a single panel in the top right.