Anything marmoreal is smooth and white or otherwise characteristic of marble, like a pale face or a tombstone. Marmoreal is an old-fashioned, fancy word for “marble-like.”
The word marmoreal comes from the Latin marmoreus for "of marble.” Marble is a type of limestone that is usually smooth and white. Marmoreal hit the scene just in time to describe all those pale white smooth faces of the Victorian age. If you’re speaking, you might say a sky “looks like marble,” but if you’re writing poetry, take it up a notch and describe it as a "marmoreal sky."