Why is Saturn called the ''jewel of the solar system?

Primarily because of its distinctive, amazing rings. Most gas giants have rings made of dust and other particles, but they're typically hard to see and are not a prominent feature of the planets. Rings like the ones Saturn has, which are large and very visible through a telescope, are incredibly rare among planets in our galaxy. The sol system is an amazing place because it boasts not just the only known life-sustaining planet in Earth, but also a beautiful gas giant like Saturn, which would likely be the first planet noticed by extraterrestrial life forms if they happened upon our system (gas giants are much easier to detect from distance than rocky planets like Earth).