A lot of people think that the diamond is the definitive gem of the engagement ring but times are changing. People are looking for ways to be different and the tradition of giving offering a diamond in exchange for a hand in manage might be turning a bit blasé. That is not to say that diamonds are, necessarily, out of style, but some people are, in fact, considering alternatives.
One alternative to the traditional diamond is the sapphire—and the white sapphire, more specifically. That makes sense, actually, since the white sapphire can have an appearance similar to that of a diamond. And diamonds are, inherently blue (or blue-tinted) so they do share many physical attributes.
The big difference—at least, in terms of appearance—is that the while they may, potentially, look alike, sapphires do not have the same scintillation that diamonds have. The term “scintillation” refers to the variety of colors typically refracted from a gem. Diamonds refract nearly the entire spectrum of colors while sapphires—even white sapphires—do not.
And that also makes sense, actually, since the sapphire is, essentially a colored gem. And this is what makes them most different. We judge the quality of a diamond by its brilliance, fire, and scintillation. We care about how bright it shines and how much light it refracts. The cut and clarity of the gem can affect those things. However, with a sapphire—even a white sapphire—there is no cut grading and no reason to judge it by its clarity.
The reason for this, of course, is that we basically judge all precious gems (sapphires, of course, as well as garnets, amethysts, opals, tourmaline, emeralds, etc) by their depth of color.
To put it simply: diamonds sparkle while white sapphires (et al) do not.
On the other hand, when a diamond is cloudy (has lower quality clarity) it is less desirable, while a cloudy [white] sapphire might actually be more interesting. Of course, the reason for this has to do with the fact that even a sapphire with the greatest clarity is not going to shine as bright—and be as interesting—as a cloudy sapphire.
All that said, though, there is one thing that diamonds and sapphires share almost identically: durability. Diamonds, of course, are nearly impervious to scratches. Sapphires are not so lucky and require more attention and care than a diamond. However, the sapphire is the second hardest substance on the planet and is actually less likely to chip than a diamond is.