Neon was an element that occurred naturally as an orderless, colorless, tasteless gas. It was the fourth most abundant element in the universe, but was extremely rare on Earth. It could be found in large quantities in liquid form on Class K planets. Neon was highly inert and forms no known compounds.
In 2267, the Galileo crash landed on Taurus II, near the Murasaki 312. The crew found the atmosphere breathable, an oxygen-nitrogen mixture, with traces of neon, argon, and krypton, not dissimilar to that of Earth. (TOS: "The Galileo Seven")
Later that year, the Galileo II was forced to crash land on a planetoid in the Gamma Canaris region. The crew found the atmosphere to also be an oxygen-nitrogen compound similar to that of Earth, again containing trace amounts of neon, argon, and krypton. (TOS: "Metamorphosis")