Stromatolite, layered deposit, mainly of limestone, formed by the growth of blue-green algae (primitive one-celled organisms). These structures are usually characterized by thin, alternating light and dark layers that may be flat, hummocky, or dome-shaped. The alternating layers are largely produced by the trapping of sediment washed up during storms on some occasions and by limestone precipitation by the blue-green algae on others.
Stromatolites were common in Precambrian time (i.e., more than 542 million years ago). Some of the first forms of life on Earth are recorded in stromatolites present in rocks 3.5 billion years old. 
Country of Origin: Bolivia.
Dimensions: 18mm to 23mm x 15mm. (may vary)
Weight: 12g to 16g (may vary)
Price are per/stone.