Yeah! It was just a quick stressor I thought of because the game reminded me of digging underground in Pokémon Diamond/Pearl.
Depending on the rarity of the shell, the player could only brush over them for an allotted amount of time or strokes. So, if the player found a rare shell and carelessly brush liked crazy, they would break it. But that does alter the vibe of no stress you have going on, which is a loved concept. Unless others say they want some stress added, I wouldn't worry about it!
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I thought of something else, so you don't have to completely alter your brushing method/relaxing vibe. Since the overall goal is to increase the player shell collecting speed you could make a "bad shell" or sea urchin or something that makes you lose shells from your bag. So, if you notice what it is before you brush it too much, you can cancel and keep looking. Sorry I know that was random, but you have a good game, so it stuck with me.