Nancy Wineman, age 13, of Sioux City, Iowa, for her question

What is a travertine?

The waxy travertines are calcium carbonate minerals, artistically molded by the fresh waters of lakes and springs. Their basic ingredients are calcium, carbon and oxygen. But the handsome pores and streaks that enhance these calcites are created by impurities, by bubbles or by embedded fragments that later decay. The basic color may be creamy white, tan or some other pale pastel tint. The streaky artwork is darker and may be etched by particles of iron or magnesium, zinc or cobalt or by varieties of other impurities.

Calcium is prone to dissolve in water and its invisibly fine fragments are carried in many streams and springs. Under certain conditions, this water is forced to dump its load of assorted chemicals and they form layers of solid minerals. Travertines are created when mineral rich lake or spring water is cooled or evaporates. These handsome minerals were named for a place in Italy where they were quarried in ancient times. We now find them around hot springs in California and many other places.