The tradition of the diamond engagement ring is, perhaps surprisingly, not a very old tradition. Diamond engagement rings were popularized in the 1930s by the De Beers diamond company. One of the marketing strategies was to have popular starlets of the time sport De Beers diamond rings in public. I do believe their strategies worked, given the current popularity of diamond engagement rings.
Although diamond rings are beautiful, not everyone wants one. Fortunately, there are many alternatives to the diamond engagement ring. A popular idea is to create a ring centered around the bride-to-be's birthstone, such as garnet (January), emerald (May), sapphire (September), or topaz (November). Some brides may even prefer to wear just a simple and elegant wedding band.
Here are a few alternative engagement rings I like:
Emerald and Diamond Engagement Ring : 14K White Gold - 1.00 CT TGW
14K White Gold Square Created Emerald Engagement Ring - Size 6
14K White Gold Medium Oval Gemstone and Diamond Engagement Ring-Garnet, size7.5
1.11Ct Emerald Cut Garnet and Diamond Ring in 10Kt White Gold-7