What engagement ring style suits you?
Before he pops the question, be sure you have an engagement ring style in mind. After all, he will likely be watching for you to drop hints on what you are looking for!
Picking the shape of your stone is very important. Make sure you pick a shape that you enjoy and keep in mind that the pointier the edges, the more problems you are likely to have with prongs needing replacement. Also keep in mind that the number of cuts/facets your stone has will affect how much light it reflects as well — shapes like princess and asscher reflect more light than say an emerald cut.
Size is important! You can create a larger look with one big solitary stone — or you can create a large illusion with a tiered effect, a larger stone in the center accompanied by stones that gradually get smaller around the band. Try on many different styles to see which look suits you best.
Anything goes as far as color and stone choice these days. Decide whether you want to be traditional with a white diamond or if you want to follow the current trends of color. Tanzanite is becoming very popular since it is considered a "one generation gemstone" that will be completely mined-out in as little as 10 to 20 years. The demand for this vibrant blue stone is very high and it may be worth a fortune to your children or grandchildren one day.
There are also options out there now from companies like Gemesis that offer lab-created diamonds. Lab-created diamonds are physically, chemically and optically the same as the highest-quality mined diamonds. By recreating the natural process, Gemesis is able to offer diamonds that possess the same excellent color and clarity as natural diamonds. The only difference is the point of origin. Gemesis diamonds are grown in a safe, humane, controlled environment using only modest amounts of energy with little environmental impact. Choosing a lab-created diamond may be something you want to look into.
Rose gold, also known as pink gold, is very popular at the moment so if you want to be different you could take a look and see if it is for you. Yellow gold, white gold and platinum are the more traditional routes.