Clarity ? the transparency of the stone and any inclusions (visible flaws)
Cut ? how well cut the diamond is (not the shape)
Carat ? how much the diamond weighs
Color can be manipulated in several ways. The diamond is shown under a colored light that offsets any yellowness in the stone or is shown against a black cloth backdrop instead of white. The setting of the stone can influence the perception of the color. A white gold or platinum setting contrasts with a yellowish diamond making the yellow cast more obvious, while a yellow gold setting doesn't make the yellow color as obvious. None of these methods are dishonest as long as the seller states the actual color grade.
However some dishonest sellers misstate the color grade intentionally or enhance it. Yellowish diamonds can be made to look whiter by touching the side of the diamond with indelible purple ink which neutralizes yellow. The diamond can also be painted to neutralize the yellow. The "paint" isn't at all obvious, can't be washed off and may take months of cleaning before it wears off.
All natural diamonds have some flaws. The diamond can be set to hide an obvious flaw on the edge of the diamond, the prong of the setting hiding the flaw. If you're buying a one or more carat diamond in your engagement ring it should be examined unset. Diamonds can have a crack or fissure filled thereby concealing it. Sometimes an inclusion can be lasered out. These techniques are not unethical as long as the seller discloses they've been used.
The weight of the diamond is measured in carats and increases exponentially. In other words a two carat diamond with similar clarity, color, and cut, will cost much more than twice the price of a one carat diamond. Some diamonds are cut to leave a bit of unpolished area on the girdle to make sure the diamonds reaches that important one carat weight. A .95 carat diamond price per carat cost is much less than a 1.00 carat diamond price per carat cost.
A three carat diamond engagement ring can vary tremendously in value depending on how many diamonds make up the three carats. A two carat center stone offset by two 1/2 carat diamonds would be costlier than a ring with a one carat center stone offset by four 1/2 carat stones, even though both rings have three carats of diamonds.
Diamonds that are two carats don't look twice as big as a one carat stone. The increase in perceived size is only about 50%. A word of warning: Be leery when someone says a diamond spreads 1.5 carats. That doesn't mean it weighs 1.5 carats, just that it looks the same size as a 1.5 carat diamond when viewed from above. That can be achieved by cutting the diamond broader but shallower.
Cut is one of the more important factors. A badly cut diamond loses its brilliance and sparkle compared to a well cut diamond. A round brilliant cut diamond is about half the size of that same diamond in the rough, or uncut. Some diamond cutters will try to eke out two one carat stones out of a rough diamond crystal by sacrificing the quality of the cut.
The best way to know that you're getting what you're paying for when purchasing your UK diamond engagement ring is to purchase from a reliable dealer that provides a diamond grading report from an independent gemstone laboratory.
