If you are looking to avoid diamonds for whatever reason, (budget, ethics, antiestablishmentarianism), I would recommend going with a precious colored stone instead. You can read my advice on that here. But if you absolutely must have a white stone there are a number of alternatives to diamonds, though in my opinion there is no comparison.
White Sapphires are natural stones. As I have mentioned before, sapphires come in many colors, white being one of them. White sapphires are always heat-treated to produce, intensify ,or lighten color and/or improve color uniformity and appearance. Just like colored sapphires, they are very hard, and would be a practical choice for an engagement ring. However, the white sapphires I have seen always appeared somewhat milky and in no way came close to matching the brilliance of a diamond, or even a colored sapphire for that matter. In my opinion, although they may be somewhat more expensive, a colored sapphire would be a much better choice. If you have your heart set on a white sapphire though, I would check out Gemvara, as they have hundreds of different wedding and engagement rings all available with white sapphires.
Cubic Zirconia or CZ is the most widely used synthetic diamond substitute and also the cheapest. When they first came on the market they sold for $250 per carat and now they sell for about $2 per carat. Basically, they are worthless. They are easy to spot because their appearance is “too perfect”. Cubic zirconia is much softer than a diamond and will show wear on the facet edges with routine wear. If you do decide to go with a CZ please, please go with something of a reasonable size. Anything over a 1.5 carats or so will just scream that it is fake.
Lab-created diamonds are man-made diamonds. Unlike CZ’s, which are meant to imitate diamonds, lab-created diamonds are actual diamond material, except that they have been grown in a lab rather than produced naturally in the earth over many millions of years. They usually have faint tinges of blue or yellow, as they haven’t yet exactly perfected the process. Most lab created diamonds are used for industrial purposes, so it doesn’t matter what color they are. They have only recently begun creating gem quality lab-grown diamonds. In the future, like CZ’s they will likely come down drastically in price, but right now they are almost as much as a natural diamond per carat, so I can’t really see why anyone would buy one. If you are really worried about the ethics of diamond mining, why not buy a Canadian mined or antique diamond instead?
Moissanite is a lab-created stone that was first discovered over 100 years ago by a French scientist named Henri Moissan on a meteor that struck earth. Unlike CZ’s, they are very durable, ranking only below diamond, but above ruby or sapphire, in hardness. They are substantially less than lab-created diamonds in price. Sound good so far? Well they could be a good choice, if you don’t mind that their sparkle is obviously different than diamonds.
Moissanites have a higher refractive index and different light dispersion than diamonds. Because of this, they are faceted differently. For most people, it will be easy to distinguish a moissanite from a real diamond, as moissanite’s heightened brilliance creates a gaudy, disco-ball effect, especially when seen in sunlight. The larger the moissanite stone, the more apparent this effect is.
Also, most moissanites, even those claimed to be “colorless” can give off a yellow or grayish hue in certain light. A high-quality colorless diamond, whether natural or lab-created, will have a natural body color that contains no traces of yellow, brown, or grey.