What rock is it?
Hi, Justyna - The rock you found appears to be amethyst, a form of quartz.
https://en.wikipedia.org/wiki/Quartz Quartz is a very common mineral. It is the last to crystallize out of magma and the last to weather. A reference book called "Geology Underfoot in Illinois" might help explain the setting where you found the rock. Geologists, like archaeologists, use context to understand where rocks occur.
https://en.wikipedia.org/wiki/Amethyst Nice specimen!