I’m looking at a tanzanite ring at my local Jared’s jewelry store.
The intention is to use it as an engagement ring… perhaps, someday.
Is that a poor idea in terms of the hardness of the gem?
This is more of an IMHO response, but it precludes your question.
It’s a poor idea to tell a woman you love her enough to marry her, but not enough to buy her a real diamond. It’s a poor idea to purchase an engagement ring before she says Yes. You can always give here an inexpensive ring and go shopping after the deal is sealed.
I’m not by any means a jewelry expert, but I think it’s not a good choice. 
Taking a look at a few references for ‘Tanzanite Mohs scale’, it’s generally pegged at just under 7 on the Mohs hardness scale, or ‘6.5-7’
You’d want a stone that has at least a solid 7 on the Mohs scale for ‘everyday wear’ use like an engagement ring, and higher would be better.
You’re assuming all women want diamonds? Can’t stand them myself. They’re boring and they have unpleasant associations with colonialism, slavery, and civil war.
He’s asking about the hardness of the gem in relation to daily wearing not to what the beliefs of his someday fiancee might be which makes it GQ not IMHO.
However OP, since your question did get a quick response I would add that from an IMHO point of view it’s better to select a ring for a specific girl than to find a girl who will appreciate your view of what a ring should be.
Unless you know she wants something other than a diamond. Some of us do.
I didn’t want a diamond engagement ring, but I wanted something durable, because I am a klutz. I ruled out everything below 7 on the Mohs scale right out of the gate.
Yes. Then you’re also not likely to get something that is not to her taste or that is impractical for her.
Not all of us need a surprise proposal complete with ring. They don’t ask for evidence of that when you get your marriage license, or at least in North Carolina, where I got married, they don’t.
Not all women want diamonds, but as others have noted, you should wait until you have a specific person to whom you want to propose and whose tastes you are familiar with. Sapphires, rubies, emeralds, and topazes are some alternatives to diamonds, and also have the minimum hardness required.
Well call me Mr Cheapskate, but my wife’s engagement ring is tanzanite, set between two small diamonds on a white gold band, and we both think it’s beautiful. In fact she chose it, after I proposed. I love the colour - we looked at sapphires too but never saw one in anything like as vibrant a colour.
She’s been wearing it for four and a half years now and there don’t seem to be any issues with the hardness - no scratches.
Of course, if your beloved is a bricklayer or a roadmender, you might want to reconsider. 
Edit: I do agree with the other responses, though - wait until you have the girl before you buy the ring.
Picking out an engagement ring before there’s a specific woman you’ve picked it out for- creepy. Most women are going to be reluctant to date you if they find out you’ve done this. The only exception would be if the ring is a family heirloom.
And what happens if you buy this engagement ring and never find a woman to give it to? You’ve wasted a lot of money, that’s what.
Wait until there’s an actual person with actual ring preferences in your life before you go buying an engagement ring. Then buy something she likes. She’s going to be the one wearing it, after all.
It appears to be about the same as peridot on the hardness scale. I have a peridot ring that I wore almost every day for about a year and it did develop some scratches.
That is the most hideous bullshit I have ever heard.
I do not wear diamonds and never will. They are ugly, a waste of money, and in many cases a chunk of pure evil.
To the OP’s question Tanzanite is more suitable for a pendant or earings.
Also to the OP’s question, all mall-based jewelry stores are horrible ripoffs, so if you wanted to buy a Tanzanite ring, I wouldn’t buy it at Jared.
And finally to the OP’s question, buying a ring for a someday hypothetical bride that may or may not like it in the possible future is beyond creepy.
If you really like tanzanite, why don’t you buy a pinky ring?
It’s not a good idea to buy an expensive ring for a lady you haven’t even met. Even after you meet her–she might prefer another gem. Or none at all…
Tread lightly. Without bricklayers there’d be no walls, and without roadmenders there’d be…a lot of potholes.
Hey, no slight on brickies and roadmenders. I simply meant that a tanzanite ring is probably not optimum for manual labour, if that’s your lady’s bag. 
I took the context of the OP to mean he’s heterosexual.
Mrs. Devil’s ring is citrine (a quartz; 7 or so hardness). We both have a loathing for diamonds both for the blood and suffering inherently attached to *all * diamonds plus a generalized aversion to over-marketing (see TriPolar’s ‘interesting’ quip). No scratches or problems yet. And I’ve it on good authority that she loves me.
Tanzanite is without question too soft and too scratch-prone (mohs 6.5) to be used as an engagement ring if it’s the sort of engagement ring that will be worn every day. Especially if the woman in question is, like me, a klutz, and knocks her hands about everywhere.
Sapphire and ruby are hard and tough enough (mohs 9) to be used as an engagement ring. So is cubic zirconia and alexandrite (mohs 8.5), with the added advantage that very pretty CZ can be had fairly cheap (My engagement ring is a CZ.) Spinel is also a tough stone, beautiful, and just at the upper range (mohs 8) of hardness that I’d recommend for an engagement ring that was worn every day. I wouldn’t really recommend something below Mohs 8 for long-term every-day wear.
If it’s only going to be worn on special occasions, and the woman is very careful with the ring, which is the case for some women, then tanzanite is probably fine.
ETA: My engagement ring is (clear) CZ through my own insistence (mr. hunter wanted to get me a diamond), because, like Rhythmdvl, I also don’t go in for the over-marketing and the conflict diamond stuff. Although if I had to do it all over again, I’d get a sapphire or at least a blue CZ, because colored stones are just more interesting to me.
Hmm, I’m going to have to have a closer look at Mrs C’s engagement ring when I get home. As I said, she’s been wearing it for over four years, every day, and never takes it off except to have it cleaned occasionally. I’m pretty sure it’s not scratched up, though.
I did exactly that. A large tanzanite stone in a gold setting as an engagement ring for Lady Chance. She loved it and wore it everyday until we married at which point she retired it. Never had a lick of trouble with it.
And it’s a pretty stone. Good luck!
I had a pearl engagement ring, which is like a mohs 2.5 - somewhere between Play-Doh and chalk, I think. I just accepted that it was going to show wear. It did have a few dark marks and dents on the underside from where it rubbed against my wedding ring, but did surprisingly well for a couple of years… until one day it just fell off the mounting. I put it away as a keepsake and now just wear a wedding band. But my point is, I imagine tanzanite, especially one that’s securely mounted, could be just fine for daily wear. It depends on the wearer, I think.
But I agree: it’s something she’ll wear every day for the rest of her life, so don’t pick it out without taking her tastes into heavy consideration, at very least.
Colophon and Jonathan Chance, it’s very probable that your wives are not nearly as big a klutz as I am. I must knock my ring finger against something hard several times every day by accident. And a lot of these warnings are geared towards not knowing whether the woman in question is like me or not
Also, I’m talking about a forty-year timescale here more than a four-year one. In forty years I’m betting it’ll probably be a bit scratched up. In four years, maybe not so much, although if you inspect it with a loupe I’ll be interested to see what you’d find. (I’d expect at least a little wear on the facet points.)