snakestone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small stone or piece of porous substance reputed to cure snakebite.

from The Century Dictionary.

• noun Same as ammonite: from an old popular notion that these shells were coiled snakes petrified.
• noun A small rounded piece of stone, such as is often found among prehistoric and other antiquities, probably spindle-whorls or the like. Compare adder-stone.
• noun A kind of hone or whetstone found in Scotland.
• noun Same as serpent-stone, 1.

from the GNU version of the Collaborative International Dictionary of English.

• noun A kind of hone slate or whetstone obtained in Scotland.
• noun (Paleon.) An ammonite; -- so called from its form, which resembles that of a coiled snake.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A kind of hone slate or whetstone obtained in Scotland.
• noun paleontology An ammonite (because of its coiled form).