Pool filter sand turning black underneath layer?

Im in the process of moving into my first apartment and i saved the tank for last. Im moving the 75gal tank in which I have had pool filter sand for about a year or so. Now I have moved around the sand before because I know it traps bubbles. But im just about to get the last bit of sand out and I noticed its turning grey or black. Didn't know what it meant. If its dirty or what. Anyone experience the same? Just want to make sure if I should use it again or not. I havent had any problems with the sand other wise.
Thanks.

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My pops said it because its getting old. Its darker in person than in the pic.

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