Give your print depth.

Make marble veins in white and ash gray filament, with dark cores on the surface and softer veins beneath it.

Choose your modelOpen a closed STL or plain geometry 3MF. Confirm its size before applying the effect.
Find your marbleAdjust vein size, branching and depth. Try a new pattern while keeping the style.
Test your filamentsDownload small cover samples and compare how much white lets your gray show through.
Generate and sliceInspect the veins inside, download one multipart model, and assign white and gray in your slicer.
Open Marble

Free, no account. Your model stays on this device. Appearance is approximate until you compare printed samples.