Ceramic resonators? Those are an alternative to quartz crystals. Their main advantage is that they can be smaller and cheaper, but their main disadvantage is that they are not as accurate as quartz crystals. So if you’re wondering how the Raspberry Pi gets its clock signal without a quartz crystal, well that is one way. Actually, looking at the schematics and reading more detail here, a quartz crystal actually is used for the oscillator.
20191109/https://en.wikipedia.org/wiki/Ceramic_resonator
20191111/DuckDuckGo raspberry pi crystal X1
20191111/https://raspberrypi.stackexchange.com/questions/74482/switch-out-the-x1-oscillator-on-a-rpi-2-3