Source for sand and gravel for 3 bucket filter

So I want to try a 3 bucket filter. As I understand it, I’ll need (duh) 3 buckets, other stuff, sand gravel, and activated charcoal. I was inspired by this link:

http://www.askaprepper.com/building-...-water-filter/

I have found food grade activated charcoal, no worries there:
https://www.amazon.com/Granular-Acti.../dp/B00KYYP2A0

But now — and I know I might be being silly here — I’m worried about the gravel and sand. Yeah, I can just get gravel and sand at Home Depot, but should i run my drinking water over it? What’s it treated/washed with? I’m looking for just pure rocks and pure sand, no toxic chemical stuff done to it

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Source for sand and gravel for 3 bucket filter