All limestones start as floors of shallow tropical seas. The famous stones of Portland and Bath were laid down 150million years ago when what is now that area of England was more like the Bahamas are today. Limestone is still being formed in parts of the tropics, where some perfectly good, hard stone is no more than 30 or 40 years old. It has even been known to contain impurities like cola bottles.

http://www.naturalstonespecialist.com/information/limestone.php Interesting article.