The term gravel (in engineering classification) means the fraction of soil greater than approximately 1/8 inch in diameter. Pea gravel is a colloquial term referring to gravel which is typically greater than 1/2 inch in diameter. Gravel may be made of essentially any kind of rock, including common types such as limestone, granite, or sandstone. For your purposes, it makes no difference what kind of rock the gravel is made of. Your local material supplier will have a range of gravel size from 1/8 inch up to at least 3/4 inch, it may be called crushed stone, it may have a local name, as long as it contains only a small amount of fine material (less than 10 percent by weight) it will work well for drainage.
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