Fun, I love unique rings and new traditions! I have two friends that asked for and received sapphire engagements rings, both in blue.
I'm not sure if you would consider a colored diamond? There are so many colors now and days such as yellow, shades of pink, cognac, black, and I think there might be a blue shade but I am not sure.
For other blue stones, you can do sapphire (comes in many different shades of blue and even pink), aquamarine or turquiose.
For green stones you can do emerald or citrine (usually a lighter green).
For irredescent blue/purple you can do an amethyst.
Not sure what your birth month is but maybe you can accent it with that stone or use it as a center stone.
Also another suggestion would be white sapphires. They use them often as diamond substitutes and are beautiful as a center stone or even accents.
Best of luck!
What about your birthstone? That is what my SIL has and it is beautiful
emerald if you love green,sapphire if you love blue
I know quite a few people who use Sapphires and Emeralds instead of diamonds. For sapphires use a silver band and for emeralds use a gold band. They will look exquisite:)
Sorry to be negative, but will you know how your sapphire or emerald or whatever you choose has be mined? You can get certified diamonds these days.
If you're opposed to diamonds based on moral reasons you should know that the diamond mining companies also mine the other precious and semi-precious gemstones. If you are set on conflict-free gems you will have to skip the major distributors (De Beers, BHP etc) and possibly buy direct from the wholesaler.
As for tourmaline, it rates 7-7.5 on the Moh's scale which means it will hold up well.
A deep blue sapphire would be lovely, and the sapphire was one of the more popular and traditional engagement stones before diamonds became common. One problem you may run into is that a lot of gemstone rings have diamond accents, and it may be hard to find something you like that doesn't have these diamond accents. If you look for lab-created sapphires (or whatever stone you decide on) you will probably find that they have cz accents instead of diamond.
Emerald or ruby would also be great choices, as they are some of the hardest (and most durable) gemstones after diamond. Don't get anything topaz because they are too soft and will get scratched and dull with everyday wear.