*raises hand* re: tanzanite being heat-treated... what does that mean?
Also, I never thought about jewelry not retaining speculative value. I suppose the whole "diamonds are a girl's best friend" thing makes it seem like that jewelry/gems are an investment/safety net.
Good question!
Now, let me preface this by saying: 95% of all colored gemstones on the market have had some manner of artificial treatment to make them more appealing. Generally speaking, this is a known quantity, and not something you need to be alarmed about.
By far the most common treatment (and it's attested in the archeological record, btw, so humans have known about this for a long time) is heat treating. You heat a mineral to a certain temperature for a given amount of time and sometimes the clarity will be improved, or the color will change or intensify. Courtesy of r/gems, here's a great side-by-side comparison of what most tanzanite looks like, before and after heat treatment:
This isn't a scam and it isn't some big secret -- it's best to assume that any given colored stone has been heat treated unless you have an independent lab report from a reputable firm saying otherwise. No, a private appraisal isn't good enough. No, the jeweler's word isn't good enough either. Heat treatment is stable (i.e. permanent) and not discernible to the naked eye, so it's not something you as an average consumer needs to worry about. It's also not a value determinant in almost any stone. Nobody gives a crap if your amethyst is 'unheated'. Nobody cares. Don't get tricked into paying a premium for it.
The majority of tanzanite is reddish-brown when it comes out of the ground, and only after heating do its lovely blue-violet tones show.
For your second question, while diamonds do retain more of their value, speaking very generally, than almost any colored stone, the speculative value of gemstones bought at retail is, like most things, effectively nil. Buy things because you love them, not because you're hoping someone will pay you more for them later -- the late capitalist need to turn every blessed thing on the planet into a casino chip is what turns people into motherfucking Daleks.
