- Maine, United States2
- Mammoth Cave, Edmonson County, Kentucky, United States4
- Mansons Mine, Bucks County, Pennsylvania, United States1
- Maryland1
- Maryland, United States1
- Massachusetts1
- Massachusetts, United States9
- Matlock, Derbyshire, England, United Kingdom1
- McNamee Mine, Bath, Aiken County, South Carolina, United States1
- Mediterranean1
- Mexico2
- Middlesex County, Connecticut, United States2
- Middletown, Middlesex County, Connecticut, United States1
- Midland, Cabarrus County, North Carolina, United States1
- Milos, Nisos, Cyclade Islands, Aegean Islands, Greece1
- Mississippi River1
- Missouri, United States2
- Montclair, Essex County, New Jersey, United States1
- Monte Baldo, Verona, Italy2
- Monte Rose, Liguria, Italy1