I knew if I took a swing at many different things, eventually I would hit a home run.
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Sometimes rocks like this are quite dirty. If they are raked around, sometimes the bulk of the dirt falls off. Otherwise they can be raked into a tall pile and washed with a garden hose. I often clean rocks where they sit by using a leaf blower. After the initial cleaning, I rake them around and do it again. This is best done during very dry weather.
Several of my customers have Japanese style gravel gardens. They are kept clean with only a few minutes labor per year. Rocks used in drip zones are given the same treatment.