Amber, which we now know is the aging resin of several different trees and shrubs, was of unknown origin to the ancients, who revered it as a great element in magic and used it often as a talisman.

Because it was found most frequently on the shores of streams, in old lake beds, or in the sea, it was often thought to be the product of a fish that was called, appropriately, the amberfish. Others believed it came from seafoam that had crystallized, or from resin put forth by certain trees. So when the artist of the Hortus Sanitatis, published in 1491 by Jacob Meydenbach, was required to portray amber, he cleverly composed all these legends and produced a foaming ocean in which an amberfish swims under an amber tree growing out of the waters. The look of doubt expressed in the glance of the fish perhaps says it best.

An Amber Tree growing out of the sea, from the Hortus Sanitatis

Mythical Plants of the Middle Ages is based on the writings 
of Ernst & Johanna Lehner and William A. Emboden.
Information provided by: http://www.godecookery.com