Karin Tditzk, age 11, of Lethbridge, Alberta, Canada, for her question:

What gives a diamond its density?

The density of any substance depends upon how much matter is packed into a certain size, or volume. A diamond is the hardest of natural substances, but many other substances are more dense. It is, of course, a special crystal form of carbon. The structure of a crystal depends on the shape and size of its basic atoms or molecules and especially upon the chemical attractions they exert upon each other. All carbon atoms, naturally, are alike in all respects.

As a diamond crystal grows, these built in properties arrange its carbon atoms in cube shaped units, linked at each corner to other identical units. The crystal is a latticework, built from particles of matter with spaces between them. Diamond is a rather loose, openwork lattice. It would be a much denser substance if its carbon atoms were crowded together, with fewer and smaller spaces between them. However, the diamond's rather loose latticework happens to form the hardest of nature's substances.