What is the heaviest naturally occurring element by atomic number found in more than trace amounts?

Uranium, atomic number 92. Heavier elements exist in nature only in vanishing quantities produced by decay or neutron capture - neptunium and plutonium occur in uranium ores at parts per trillion - so uranium is where the periodic table effectively stops being something you can dig up.

It was named after the planet, discovered eight years earlier, and for a century and a half it was used mainly to colour glass and ceramics yellow-green. Uranium glass still fluoresces under ultraviolet light and was sold as ordinary tableware; Fiestaware's orange-red glaze was radioactive enough that the range was discontinued during the war when the government wanted the uranium.

There is also a natural nuclear reactor. At Oklo in Gabon, about two billion years ago, uranium deposits with a then-higher proportion of the fissile isotope, infiltrated by groundwater acting as a moderator, sustained fission reactions on and off for hundreds of thousands of years. It was discovered in 1972 when French technicians found ore with the wrong isotope ratio and worked out why - and it is studied as a natural experiment in how radioactive waste migrates over geological time.