What makes a diamond yellow?
Pure carbon (a pure diamond) is absolutely colorless. Small trace amounts of other elements change it's appearance. It's nitrogen within the carbon atoms that cause a yellow tint.
This is true with all gemstones. In their pure state, most gems are colorless until trace elements are introduced.
How are Proportions graded?
If one were to look at any diamond grading certificate and look at the different gradings on proportions, they'd find parts of the diamond like Culet, Girdle, Finish and Symmetry.
The Culet (cue-let) is the pointed (but not always) part at the bottom. Some culets don't come to a sharp point and may have a tiny facet.
The Girdle is the very edge around the circumference. Some girdles are faceted, rough, smooth, very thin (knive edge) or wavy (between very thin and quite thick).
Finish and symmetry refer to how well the facets line up and how well the stone has been polished (drag marks).
» GIA Grading & Reports: Anatomy of a Report
» How GIA Grades Diamonds: View Video
Do these things really matter?
Yes. If you have 10 power magnifying eyes or you often carry a jewelers loupe with you at all times.
Otherwise, it only matters on paper. You'll never see the culet or whether a facet lines up or even a wavy girdle with the naked eye.
So if it's imperative that you buy a diamond with EX/EX (excellent proportions/excellent finish) do so for your own piece of mind and comfort because not one person at the next office party will ever know.
We call this a 'good night sleep factor' because if it helps you sleep better at night, that's all the reason you need.
We carry most all grading levels because it doesn't matter to us, it only matters to YOU and that's what counts.