http://www.forrestrace.com/ring-size-3/

Celebrate Your Engagement With The Perfect Ring
The practice of giving a ring upon betrothal dates back to ancient times. However, it was a far cry from what is sold today, normally consisting of a plain metal band. The first known instance of a diamond ring given to an intended bride was quite literally fit for a queen. It was given in 1477 by the Emperor Maximilian to Mary of Burgundy to celebrate their engagement. About two hundred years ago, the supply of diamonds became more plentiful and the Industrial Revolution brought more wealth to the average person. The combination of factors allowed more people to buy diamonds, once within the reach of only the wealthiest.
Solitaire engagement rings are the most popular variety sold. The term technically refers to a ring with a single stone, regardless of cut. However, a round cut is the most common, although there are many different ways to mount it. The round cut is considered to be optimal for enhancing the reflective properties of a diamond.
A trilogy setting is a common variant of the solitaire. These have a larger stone in the center and a smaller gem on either side. While these may all be diamonds, in the last few decades jewelers have produced several variations. The center gem can be an emerald, ruby, or sapphire, or a birthstone can sometimes be used.
Many modern brides long for an antique engagement ring. Since they only have to be more than 50 years old to qualify, the majority are diamond rings quite similar to modern ones. However, during the twenties and thirties, the art deco movement led to the design of some novel mounts and the use of colorful stones. Rings that are older than a century or so may not even have any diamonds at all, especially those that date back to the 1600s and earlier. Most of the rings of this age use other gems, and many also have very elaborate mountings.
Princess cut engagement rings have been gaining in popularity in recent years. These diamonds are cut into a square shape and have a flat top. They are sometimes mounted flush with the band, but the typical mount places the prongs at the corners of the stone.
Emerald cut stones are rectangular in shape. Typically they have facets along each side and a top that is flat. The marquis stone is fairly round, but it will taper into a point at each end. Pear shaped cuts are rounded at one end and sharply pointed on the other end. They are more often made into pendants, earrings, or set among additional stones since their shape is not considered an attractive solitaire.
It should come as no surprise that some of the modern ideas about diamonds are the result of advertising campaigns by jewelers. For example, the notion that a prospective groom spend two to three months’ salary on an engagement ring was a promotion by a major American jeweler. Nevertheless, diamonds have remained the most popular precious gem, and most women prefer diamond engagement rings. Size, cut, and mountings may, however, vary a great deal. But now that diamonds are plentiful, varied, and affordable, almost everyone can achieve her goal.
How to Pick a Diamond : Ring Size Advice for Engagement Rings