A Crystal oscilator regulates the speed of CPU's if you have a 50mhz 060 you can use a 50mhz cystal or overclock the chip with a higher crystal. I never understood completely but if you look on your A1200 motherboard, next to the ROM sockets is a Silver chip. Mine says it is running at 28.37516mhz. That is a crystal oscilator. For a more detailed description of a crystal oscillator goto http://en.wikipedia.org/wiki/Crystal_oscillator. It doesn't explain the relevance of crystals with Amiga but essentially it regulates the speed of the CPU.