Uranium

Uranium, U, 92, is the heaviest naturally occuring element, and is very well known as a nuclear fuel for explosives and reactors.

In truth, uranium is quite common in nature, and can be found almost anyplace on the planet, however, the only useful part of uranium to humanity tends to be the bits that can explode or heat up, and that's the isotopes U-234 and U-235(numbers refering to the total atomic weight, see Radioactivity for more info). Unfortunetly, uranium you dig out of the ground has less than 2% of these valuable isotopes, the rest being U-238, which is fairly useless.

Because isotopes have very nearly similar chemistry, it is very difficult to separate them. The main method used today is converting natural uranium into uranium hexafluoride, which is a gas, and then spinning the gas very fast, causing the slightly heavier atoms of U-238 to move to the outside, and the lighter U-234 and U-235 remain in the middle, where they can be taken out, and purified again, and again, and hundreds of times again, this continues until you get pure enough U-234 and 235 to be considered useful. For bombs, you need about 90%, which is amazingly pure for isotopic purification, and you only need higher than about 30% to be useful in reactors. This entire process is extremely expensive and difficult to engineer, which is quite lucky, because otherwise our entire planet would be perpetually in nuclear war.

Before all this bombs and radioactive stuff was known about, uranium was used as a peaceful dye, used to color glass and ceramics. Certain uranium oxides mixed with glass give it a nice green color, and UO3 oxide can be used as a glaze for a beautiful bright orange. Uranium nitrate can also be used for photographic prints, known as "uranotypes" which came out orange and white(instead of black and white), but silver nitrate proved much easier, so it was never a popular method.

These are my samples!