This is probably just something really common and boring to most people, but I was intrigued so I thought I might as well ask now I can. When I was in Portugal (I don’t know specifically where we were) when I was 6 (I’m now 15) I found some rocks on a beach that I thought were really interesting. They ended up coming home with us after the holiday, and I just rediscovered one in the house. This particular one doesn’t, but some of them I found seemed to have some sort of crystal inside?? It reminded me of those quartz crystal dragon eggs (I don’t know what else they would be called?) except it was only a few tiny bits where you could see into what seemed to be a hollow rock besides the crystal or whatever it was.
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From your picture, your rock appears to be made up of the mineral quartz (silicon dioxide), which is a very common mineral. Quartz may be coloured but is commonly white or clear and often tumbles to a smooth surface like this on beach pebbles. If that is the case, you won't be able to scratch it with a knife.