What does your girlfriend expect in terms of an engagement ring? Have you seen her point out engagement rings belonging to her friends before, or maybe rings in a jewellery shop window?
She will have a certain minimum size that she deems is acceptable, that she has arrived at from what her friends have, what she sees in the media and what she knows about the High Street cost of diamonds.
Knowing what you now know about the factors that influence the price of a diamond, you know that size (carat) of the diamond is only one part of the price equation. As diameter of the diamond is the most obvious outward appearance it will probably be the most common thing that your girlfriend comments on.
Once you know roughly what diameter diamond she expects you can then set about matching that diameter with the correct proportions, clarity, colour, and category of retailer so as to pull her perfect diamond ring back into your budget.
If you also spend time educating her about diamonds, it is highly likely that she revises what she wanted in the first place.
In this way your girlfriend gets what she wants, you get what you want, and she is massively impressed by how much effort you have put into researching this most important of purchases and how far you are willing to go to get her the perfect diamond!
