Marbling was a technique of painting wood (or other materials) to look like marble. It was commonly used on baseboards, trim, and columns through the 1800s.

Marbling was discovered on the staircase at Adena. Paint analyst Frank Welsh took a paint chip from the stairs, and under his microscope saw the marbling. Further, the chip revealed that the stairs had never been stripped of paint, indicating that the original marbling was still there under layers of paint and varnish. OHS staff carefully removed the paint on some of the risers and did indeed uncover the original marble pattern, seen on the lower riser in the photograph below.

As careful the restorers in the 1950s were, amazingly, they completely missed this detail.

Chemical analysis confirmed that the paint was from the early 1800s. OHS staff marbled the wood work on the stairs based on the exposed pattern. The original marbling is still preserved under layers of paint.

Marbling is allied to graining, and both are generally performed, when in the best manner, by painters who confine themselves entirely to this branch of the trade. There is much skill and ingenuity shown in imitating the various marbles and porphyries by the study of good slabs of these materials; and when the selection of colours is judicious, the effect is certainly extremely rich, and much preferable to the quantity of cold white once almost universal in ordinary houses. But when the imitations of wood or marble are badly performed, the effect is, on the contrary, particularly disagreeable.

(From, Thomas Webster, An Encyclopaedia Of Domestic Economy
(New York, 1845), page 76)