Density is a ratio of a substance's mass to its volume. For instance, density explains why a certain amount of lead feels heavier than an equal volume of salt. Diamond is amazingly dense given the low atomic weight of carbon. At 3.51 grams per cubic centimeter, diamond is much more dense than graphite, which weighs in at only 2.20 grams per cubic centimeter. This comparison offers an important clue to diamond's origin: the fact that diamond's carbon atoms are "squeezed" together tighter than in graphite, which forms near Earth's surface, implies that diamond is formed under high pressure conditions. This concept was corroborated by the experimental synthesis of diamond at high pressure and temperature illustrated on the graph below.

This simplified diagram shows the conditions of pressure and temperature where diamond and graphite will be the stable forms of carbon. The points show the conditions at which diamonds were first grown by the companies ASEA and General Electric in the early 1950s. Temperatures are in Kelvin--subtract 273 to convert to degrees Celsius.

This magnitude of pressure is difficult to comprehend. For example, the pressure of 55,000 atmospheres necessary to make a diamond at 1400 degrees C (orange hot) would require:

• 10 Anighito meteorites (10 X 20 metric tons) resting on a penny.
  
• The Eiffel Tower (7000 metric tons) resting on a 5 inch plate