First, the most important thing about an engagement ring is its symbolism. The ring itself is meant to symbolize something beautiful, rare, and precious-- like the love the couple hopes to share for life. It doesn't have to be costly, and it doesn't have to be a diamond ring (just take a look at the royal family-- the Queen Mother, Queen Elizabeth, Princess Anne and Princess Diana received sapphire engagement rings, and Princess Sarah Ferguson, a ruby). It simply has to show thoughtfulness and caring, and incorporate appropriate symbolism for the particular couple.

For example, there are many colored gemstones that have historical associations to love. Red stones symbolize the heart, and thus, passion and a zest for life. With red, one instantly thinks of ruby, but the same symbolism can be conveyed very inexpensively with red garnet, or with a little larger budget, red tourmaline, or for a little more, red spinel. All natural gemstones, and all beautiful.

We can find similar choices for green (which symbolizes faithfulness and continuity) where choices include not only emerald, but green garnet or green tourmaline; for blue (which symbolizes spirituality and purity) in addition to sapphire we have blue spinel, iolite, and blue tourmaline. You might also combine whatever stone you choose with a small diamond on each side, thereby completing the ring with the symbolism of diamond, which, because of its unrivaled hardness was believed to be indestructible.

And lets not forget the gem that has been associated with love and with the wedding itself longer than all others, the pearl; one might seek a natural pearl in an antique or heirloom piece.

There is also a tradition of using a birthstone in the ring--hers or his--something connected to the couple in a very personal way, perhaps also combined with diamonds and their symbolism.

White Sapphire

Scientists have, for many years, sought to find a more inexpensive way to recreate the beauty of diamond, and have come up with a variety of alternatives. One of the earliest of these was White Sapphire, which has been used as a diamond substitute since the beginning of the twentieth century. Though this stone displays many of the same qualities of diamond, it fell out of popular use in the 1940s. Because of this though, it remains highly prized today, as the jewelry it graces is now antique, which gives white sapphire a charm all of its own.

Moissanite

The most recent diamond substitute developed is Moissanite, a substance originally discovered in a meteorite, or shooting star, that fell to earth. While moissanite is used in jewelry as a diamond substitute, many experts feel it to be even more beautiful than the gemstone itself. Moissanite has a similar pattern of refraction to diamonds, however unlike diamonds, it is hexagonal -- which, means its facet edges are doubled when compared to diamonds, producing a stone that is twice as glittering as its counterpart. As those in the know would say, moissanite offers a gem with twice the fire of a diamond. While the limited availability of moissanite makes it more expensive than cubic zirconia, it is still considerably cheaper than real diamonds. Choosing moissanite will give your ring twice the sparkle at a much-reduced price.

Ruby

An engagement ring is a special piece of jewelry that should ideally reflect something of the bride-to-be and the love she shares with her future husband. Increasingly, women are choosing gemstones that are more personal to them than diamonds, selecting perhaps a birthstone, or a gem in a favorite color. A popular alternative to the purity of the diamond is the passion and lifeblood represented by a Ruby, which is sometimes called the "king of gemstones". Sensuous and fiery, it is the perfect symbol of the intimacy shared between the future husband and wife.

Emerald

Another evocative choice is the springtime green of Emerald. The color of Venus, the ancient Roman goddess of love, emeralds are an enduring symbol of love, beauty and youth, making them the perfect expression of a long-lasting love that will remain as pure and true as the day on which it was first celebrated through the act of betrothal.

Jewelers: Do you deal primarily in diamond-alternatives? We'll advertise your company here for free!

On a lighter note, you could always plop down $25 dollars and get one of these:

Srcs:
http://jewelry.about.com/cs/engagementtrends/a/stone_options.htm
http://www.jewelrymall.com/newsletters/050926.html
http://www.newyorkweddingring.com/diamonds/buy-lab-created-diamonds.html
http://www.newyorkweddingring.com/diamonds/moissanite-engagement-ring.htm
http://www.newyorkweddingring.com/diamonds/gemstones.htm