Diamonds do not last forever, contrary to familiar advertising slogans.
Together with graphite and fullerene, diamonds are one of the three crystal variants of carbon. They possess a cube-shaped crystal grid and in their pure form are completely transparent. However, natural diamonds frequently contain impurities, and it is to these that they owe their characteristic colorations.
In addition to being produced by meteorite strikes, diamonds are created in the Earth's interior at depths of up to 150 kilometers (c. 93.21 miles) as a result of the high pressure and high temperatures (up to 3000 degrees Celsius, or 5432 degrees Fahrenheit) to be found there. They are pressed toward the Earth's surface through volcanic vents, and there accumulate in sedimentary rock. With a hardness grade of 10, diamonds are the hardest naturally occurring mineral;* they are at the same time the best heat conductor among solid bodies. The Greeks called them adamantos, "the invincibles".
The seeming imperishability of diamonds gave rise to various superstitious speculations. People used to believe that diamonds could not be damaged by fire or iron, but only by the blood of a he-goat. This myth was not disproven until in 1773, when Lavoisier succeeded in turning a diamond into carbonic acid by means of a burning glass. You can easily replicate this process in a little experiment, thereby banishing the theory of the ►everlastingness of diamonds into the realm of popular misapprehensions.
For this you need a quartz tube, a standard Bunsen burner, an oxygen bottle, and a small diamond, the last of which you can surely borrow from one of your relatives or friends. Place the diamond in the quartz tube and flood the tube with oxygen. Then heat the exterior surface of the tube carefully with the burner (a gas stove would work as well). The diamond will light up brightly and start to shrink until it completely disappears. It will not even leave any ashes behind; the combustion product is pure carbon dioxide. Depending on the value of the diamond, and the temperament of the relative or friend, you may have to go away on a longer or shorter journey after the experiment.
* Only densely packed diamond-coated nano sticks are harder than that; but they are artificially generated.