These stones called Stromatolites come from the Mary Ellen Mine in Minnesota, USA. A Stromatolite (literally, 'layered rock') is a solid structure created by single-celled microbes called cyanobacteria (blue-green algae). These bacteria are the ones originally responsible for releasing oxygen into the Earth's atmosphere, making possible the multiplicity of life on our planet. Stromatolites are very ancient stones, originating billions of years ago, within the very organic processes that led to life on Earth as we know it. These are the only gemstones I know of that are so thoroughly interwoven with the web of life, especially in its earliest stages. They are also magnetic, extending their vibrations into the physical spectrum of electromagnetism. On the spiritual level as well, Stromatolite is one of the bridges between the mineral realm and organic life.
Pictured are examples, (stones sold individually) shapes and color may vary somewhat
 
Small 20mm
Medium 25-30mm
Large 35-40mm
 
Stromatolite Healing Properties and other items