Yes Marble promos do exist. The 1962 Ford at the top of the page is mine.
Promos are made from colored plastic. The color is mixed in before the promo is molded. When changing colors a few runs must be made until only the new color is in the mold. These runs result in marble looking promos as the old and new colors mix. AMT used to assemble these, but they were only sold to AMT employees. Most were used up as toys, but at least one woman was smart enough to save some. These were put up for auction by a Detroit auction house.
Very few survive today. Plus of course all are 1 of 1. The beautiful and impressive collection below is from Paul Cella. Sorry none are for sale.