Plasticy waxy substance from gravel

[xzied]

Hi Guys,
Anybody had experienced purchasing"classica natural gravel" distributed by yi hu?
I've rinse it more than 20 x and strange thing is not dirt, but plastic and waxy like substance coming out from the gravel and there's a layer of oil when I rinse it. Wonder if I should still use it after it's cleaned (if it's ever gonna be cleaned)

got some pix here

https://www.flickr.com/photos/129290...57655373722678

[DragonFireSG]

Does it melt when heat is applied to it? The gravel might have been treated with paraffin wax to make it look shiny...

[xzied]

Hmm knowing that it is for aquarium usage, I wonder why they want to do that. Anyway I had done some googling and it seems like it's not uncommon.
I've bought similar gravel with different brand and I had the same issue. I had washed it many times until all the waxy and oily residue disappear. So far it doesn't seems to have any effect on my danios.