Avodah Zarah 50 - The Worship of Mercury (Hermes)
Markulis was the name given to Mercury, and it meant "the one whose name was changed to scorn." This deity was depicted by the figure of a man with an upraised arm, and placed at a crossroads to direct travelers. Often, it was symbolized by a heap of stones - minimally, one stone perched on top of two, crudely representing head and two arms. Passing idolaters would worship this idol by tossing stones at the heap. These additional stones were forbidden for benefit, either because they were offerings to the Markulis, or because they were an enlargement of the original Markulis.
What is the law in regard to the stones found in the vicinity of, but unattached to, a Markulis? Rabbi Yishmael says that three stones found one next to another are forbidden - because they represent a miniature Markulis - but two are permitted. The Sages, however, say that the idol worshippers do not make miniature Markulis, rather, those stones that appear to be with it are forbidden, others are permitted.
Art: Richard Tongue - A Rocking Stone on a Coastal Cliff