Bonita Bandalos, age 12, of St. Paul, Minnesota, for her question:

What causes the dark striped effect in white marble?

Those smoky stripes and streaks in waxy white marble are embedded bits of sooty carbon. They were trapped there long ages ago, perhaps when the original limestone formed layer by layer on the floor of some ancient sea. Or they may have been trapped later, when heat and pressure processed the limestone into marble. The carbon, most likely, is fossil material from long gone plant and animal life. The original limestone was a sedimentary layer of miniature seashells. Upheavals in the earth's crust rearranged its mole¬cules and changed it into waxy marble, which is a metamorphic mineral.

Bones and seaweeds may have sifted down and become sandwiched in the layers of limy deposits. Later, they would be charred to carbon by the heat and pressure needed to meta¬morphose the limestone into marble. Other fossilized debris that may have mixed with the limestone later also would be charred. The fierce pressure needed to metamorphose the basic mineral would pull the embedded bits of carbon into the dark decorative streaks that occur in many white and colored marbles.