This is a unique stromatolitic form I first saw only recently. The sphere-shaped bacterially formed ball (often called an algal ball) surrounds the still intact host rocks upon which the colony grew in ever-expanding bands. Such forms are often called Oncolite, and are typically found in ancient lakes. The balls are also typical of younger stromatolite owing to the vast number of life forms that would feed on the bacteria as compared to the Precambrian when stromatolite was at its zenith on Earth.

This specimen has been cut along one traverse plane and highly polished to show the intricate detail on two balls and half of another, while the remainder remains in its natural form.

Of interest from a scientific perspective is the young age. This dinosaur-age, Cretaceous stromatolite was formed long, long after stromatolite-building organisms had to compete with other life forms, and hence had become uncommon on earth similar to modern times. The bacteria that formed this fossil had remained little changed during the preceding 3500 million years, dwarfing the short time span during which dinosaurs walked the earth and then quickly disappeared.

This is a very unique and desirable specimen, from a time when stromatolite had already become scarce in marine environments. While I'm loath to use the term rare, I doubt I'll be able to obtain more of this marterial that comes from a unique locality.