Heart shape stones or rings express the love and affection the couple has for each other, if used during engagement. It is pretty special if the bride has not picked it up and the groom-to-be present this, to her surprise, and makes her feel super special. The message the ring carries will surely bind their hearts together.
First of all, you should discuss about stones and diamonds, even if casually, with your partner before you decide to buy one. You can go window shopping and browse the catalogues together. You should listen to what she has to say. You should also learn which base metal she prefers, the choices being white gold, yellow gold, platinum and more.
Budget is another big consideration before you purchase your ring. Don't buy any ring in haste as you can probably get a better ring in that budget. You shouldn't buy a ring on credit card as if, in the most unfortunate case of you not being able to pay the bill, the most prized possession would be snatched away from your wife. So, it would be prudent to try out something which you can afford and which is good quality within that budget.
You must have a fair idea of what type of ring she prefers. There are completely contrasting styles of diamond ring designs, available for the same budget. You can get a solitaire as well as pave design ring for the same price. So a lot depends on what you choose. You can get so many different colored diamonds, yellow, pink, blue, green and more. Then there are the cuts, round brilliant, princess, square, cushion, asscher, emerald and more to choose from. It might be that she always wanted a pink heart shaped diamond and not the large round brilliant you are planning to give.
The most important thing which you need to consider is the personal choice of the bride-to-be. You can note the type of jewelry she wears and that would give you an idea of the ring she would like. You can check whether the white gold appeals to her more or the dazzle of yellow bright gold is more likely to win her over. The ring doesn't need to be very expensive, but it should reflect your love for her. Choose a heart engagement rings for her which really represents your heart which you are handing her over.