To Steve and all, Steve noted the beauty and liveliness of Tanzanite.
And, he noted it's unfortunate lack of durability. I've repaired
more Tanzanites than any other gem type - including Emerald. So
Steve is wise to look at alternatives.
However, there are few that come close to the look of Tanzanite.
As Mary and Ron mentioned in earlier posts, Iolite at its very best
could come close. Though in the durability category, especially with
the long term wear that an engagement ring would endure, Iolite
would need to be re-polished from time to time as well.
So, this puts the likely purchase directly in Sapphire territory.
With the wide range of Sapphire color possibilities, Steve you are
certain to find a color that will please the wearer.
You mentioned that the desired size/shape is 16 x 8 mm marquise,
which is approximately a 3.5 to 5 carat stone. When your search
moves into Sapphire, you will discover that the best colored rough
is cut into ovals, antique cushions, pear shapes, occasionally
rounds and a few emerald cuts. The marquise cut by nature is
unlikely to enhance the look of a Tanzanite look-a-like Sapphire.
And, finding a large marquise may take a long time, let alone have
it be the 'right' color.
If your heart is set on the 16 x 8 marquise I wish you good luck in
your search. I feel it is worth the effort to reconsider shape
before you commit to the project in its current phase.