I have seen “gem filters” for sale when searching the internet. Do these filters really work or is this just some way to get money out of people?
Do you mean like a gem identification tool, or a gem water filter? 
No, the filters look like square pieces of colored plastic with cardboard surrounding them. You put the gemstone on the included viewing surface, shine a light on the gemstone, and hold the particular viewing card up to your eye and look through it at the gemstone. Compare what color you observe with the included chart. Hanneman Gem Filter Sets. Examples: Aqua filter to differentiate Aquamarine from Topaz. Use the filter to see blue gemstones and their corresponding color. Ruby uses green filter, etc…
I don’t know if this is a valid tool or junk.
Ah. 
I was looking in the Gem-A shop recently and saw them there: https://shop.gem-a.com/gemmological-instruments/spectroscopes-and-dichroscopes.html So, I would say they are credible.
Thank you so much for answering my question. It is very much appreciated. I might consider purchasing a set. I go to estate sales and auctions. Any thing to help me out that’s portable and simple is worth considering. I wouldn’t let something like this replace my other gemological tools. That would be foolish and no fun.
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