The jewel of Buddha obviously refers to the historical Buddha. A real person who lived and died in India over 2,500 years ago. Born a prince in a luxurious life and set to inherit the kingdom, he was suspicious of the setup and yearned to see more of life. Old age, sickness, death.
Started him on his quest and stayed with him the whole time. First noble truth is suffering. We can't turn away from it. We have to acknowledge it, go towards it, understand it.
As a teacher, the Buddha built a unique description of how our experience works. Most notably to me, he taught that the world does not require a god or a set of gods in order to function. There are lots of implications of this, but that in an of itself is an incredible insight. So the jewel of the Buddha is the jewel of the teacher, the wisdom holder, the sage - the person who goes out on a lonely and difficult journey and comes back with some valuable wisdom to share with us.
So more broadly, the Buddha jewel symbolizes how precious our teachers are. Authors of important books. People who taught us important things growing up. Jim, Jim and Catherine. You all! The person in your life or on the bus who teaches you something valuable whether they mean to or not. All these teachers are represented by the jewel of the Buddha.
Most broadly, it's said that everyone in fact every living thing has Buddha Nature. This is an extremely large topic but just to mention it here... Every living thing has or is, in our nature, the Buddha. So naturally there are meditation practices where you can visualize everyone you meet in their fully blossomed, fully enlightened Buddhahood - with all obscurations cleared - everyone in the world fully awakened. That would be precious!
That's just a very quick introduction to the Jewel of the Buddha. Let's meditate!