- London Grove, Chester County, Pennsylvania, United States2
- Lord Dudley's Mine, Dudley, West Midlands, England, United Kingdom1
- Louisville, Jefferson County, Kentucky, United States1
- Lubietova, Banskobystricky, Slovakia1
- Lycoming County, Pennsylvania, United States1
- Lyme Regis, Dorset County, England, United Kingdom1
- Magnet Cove, Hot Spring County, Arkansas, United States1
- Magnet Cove, Hot Springs County, Arkansas, United States4
- 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
- Middletown, Middlesex County, Connecticut, United States1