Technetium, sodium lamps, indium oxide

The first synthetic element was technetium. It is from the greek word for artificial, "τεχνητός". (teknitos). The longest known half-life of a technetium isotope is 4.2 million years, so any techenetium when the earth was formed would have long-since decayed, and there isn't a natural process on earth that replenishes it.

Shiny, radioactive, tarnishes slowly in moist air

I would hold this in my hand for a moment, even with it's emitted alpha particles destroying my skin. There is something mysterious about a metal which does not naturally exist on earth.

Another isotope of technetium is used is medicine for its two useful properties: emission of gamma rays (for medical imaging), and a half-life of only 6 hours.

Consider low-pressure sodium lamps, as in orange streetlights:

Sodium-lit snow, my second-favorite weather after thunderstorms

Low-pressure sodium lights contain a vacuum with gaseous sodium atoms inside, and a high-voltage electrical current is run through it. This trick can be used to get light out of any element. Along with being a good way of producing specific colors, it is also very efficient at producing light relative to ordinary light-bulbs.

The sodium light before it is turned on contains some neon, argon, and solid sodium. Initially the argon and neon will be excited to produce a dark pink glow, it will then turn orange as it is warmed and the sodium evaporates.

Something which helps this process is the coating of the outside glass with indium tin oxide, typically In₂O₃ combined with 10% by mass SnO₂. This reflects infrared (heat) while allowing visible light to pass through.

Indium tin oxide also has the very valuable property of being able to conduct electricity while transparent, so you are likely to come across it in various displays, such as computer monitors, televisions and phones. Indium is a very rare metal, and its use in conductive conductive coating has caused it's price to rise rapidly in the past decade.