Not that I'm an expert on spinels, but natural spinels are untreated (whew!) and free of visible inclusions. However, spinel is commonly available through synthesis, so you want to make sure that you have paid for what you got. This is where I stop and a gemologist with proper equipment comes in; some syntethics are very easy to identify, but some recent processes make ID quite difficult.

The fact that your ring has some age should make it fairly easy to spot a synthetic, so if the jeweler you bought it from has a good reputation and he/she has told you it's natural, it probably is.

That leaves colour, cut and condition (i.e. chips etc.). Plus the carrots.

Colourwise, the purer the red hue, the more valuable the stone. An orangy/brownish tinge is inevitable, but the lower it is the better - value-wise; having said that, personal taste is king, and I like the brownish reds as much as the purer ones. If you look at blues, same thing except that the tinge is grey. Most greens (and all large greens) are synthetics.

Cut is easy. Or extremely difficult. I don't think anyone obsesses about "ideal" cut in coloured gemstones as some people seem to do in diamonds, but I would at a minimum check that the stone is cut symmetrically, it is evenly coloured, it shows no obvious windows or extinction points and it is lively and sparkly as much as its RI and body colour allow - David's sapphires are excellent examples of this.

Condition and size - I don't need to tell you about these, except to note that red spinel can be found in very large sizes, but these stones are exceptional and very valuable.

Lovely ring, Mikla! 

I'd have to agree with OMC, natural spinel is just like any other colored gemstone, COLOR is of primary importance - purest color is most highly sought after - then, all the other factors that OMC mentioned come into play. Pedigree can be important as well, Burmese and Ceylon spinel is highly regarded, and pinks from Africa are ok. But, since yours is an estate piece, I doubt anyone really knows the stone's origin.  All the reds, pink reds, purple reds and orangy reds, or flame spinel, are highly desirable especially when they have very little or no secondary hue or color modifier, like grey. And, as you described your stone to be "completely transparent," the best spinel is one with exceptional crystal.

I hope that you get years of enjoyment from your ring!