Why does my girlfriend have a rock in the shower?

Every time I use her shower I'm so confused there's always this weird oblong rock next to her shampoo and stuff and I can't seem to find any possible use for it like I tried it on my skin and it hurt so it's not for that. Is it for hair? Do girls put rock in their hair? What the hell
Update: It's not a dildo it's literally a scratchy oval that cut me
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