Jett is from the English word jet — the deep-black gemstone made from fossilized wood; also linked to Joan Jett's stage surname. Jett Black — Australian rock band. The gemstone jet has been used in mourning jewelry since Victorian times — Queen Victoria's mourning jewelry following Prince Albert's death (1861) sparked the major jet-jewelry industry in Whitby, Yorkshire.
Jett does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Jett reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.