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Pulled these drives from an old computer.
Wondering what the platters might be made out of.

Dont know, but I opened a server hard drive with about six or eight platters, gold colored which turned brown quickly once opened.. Seems they were sealed in a gas which protected them from normal air

Here's a post with more detail about old 5.25" hard drive platters.

viewtopic.php?f=4&t=19199&p=34525&hilit=hard+drive+gold+colored+platters#p34525

Older MFM and ESDI drives can fetch good money when working, and even fairly considerable money untested (they can be re-worked if the stepper motor has failed). It makes me sad to wonder what computer you pulled this from and it's fate....
For future reference. This has pretty much always been the case, but there has been a recent movement/crusade to save these drives.
But since it already looks like the T-1000 after being "Hasta La Vista'd, Baby!", the short answer is that the composition can vary widely, especially with older platters.
Newer platters are a very thinly coated Palladium over metal (super thin, so don't get excited). Older platters that I have come across could be nonsensical, brass or even glass, and the coating is probably not gold, even though it appears to be a similar color.
It's more than likely a discoloration, but you can verify with an acid test, or if you have a coin store nearby, they can zap it with XRF to see what the outer material is, especially if you didn't want to destroy it in the process.

I was so focused on recycling the computers I had that it was to late when I realized I should have tried to see if the computer still worked. I even think it was an IBM. The drives were labeled Mayo Clinic and one of them had been sent into a data recovery company.
Will know better next time.

It's probably a good idea you wrecked them if they came from Mayo Clinic (HIPAA regulations) even though these drives are so old, doubtful there's anything there anyone would care about.
You're incredibly lucky they just gave them to you, places like that (especially hospitals and clinics) usually opt to pay a company (i.e. Iron Mountain) to destroy the hard drives and they keep those records on hand in the event of a lawsuit.
That said, the computers these came out of were probably IBM 5150/5160 or something similar, AT/XT models and can fetch pretty good money, even for parts.

The left one looks very similar to an Apple computer's drive I had until recently. I think the prices on similar drives I saw online were not that substantial but I don't like to resell older drives whole anyway as I have no means of wiping any data on them.

I was sad there were no magnets but seeing this type of drive for the armature for the first time was cool.

Here's some pics:

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