Extremely subtle color differences can be very important to determine rarity of loose diamond, but are never a critical factor in beauty.
Most of the GIA Certified Diamonds sold today come from Africa, but Australia and Russia are suppliers of uncut diamonds as well.
Usually the gold engagement ring rough will be cut into whichever shape that requires the least amount of sacrifice. Marquise and oval shapes should have a length thats exactly twice the size of the width. If they are too narrow or too wide, the stone will loose light and appear too dark.