Nice to see the distinguished Mr. Wise posting here. To amplify on what he said, the separation of ruby from spinel is nearly trivial. If you are any good at visual optics, the “rainbows” seen in ruby will be somewhat doubled due to double refraction, whereas spinel, being singly refractive, shows only one ROYGBIV rainbow that is not overlapped with another one. If you place the gem on a typical cell phone or laptop screen and view it through a polarized sunglass lens, you will have a polariscope and can do the test for SR vs DR. Ruby=DR, Spinel=SR. Synethtic spinell may show a patchy ADR (anomalous double refraction) which makes it hard to distinguish from the doubly refractive ruby. Also, ruby is dichroic and should show two slightly different shades or red, whereas spinel shows only one shade no matter which direction you look at it in.
I should clarify that the above will separate ruby from non-ruby, but an SR red stone could still be either spinel or garnet. UV fluorescence of red spinel and magnification of inclusions will separate spinel and garnet, as will SG for some garnets. A little gemology knowledge goes a long way in this case. Hope this is of help.
Also, there is often (not always) a brown color modifier for red garnet, but not usually for spinel.