Quick and Easy Buying Guide
Carat weight: 1 carat = 200 milligrams = 6.5 mm diameter. Doubling weight doesn't double diameter.
Diamond clarity: FL/IF/VVS/VS = super expensive, near perfect. SI = best value if you can check a photo for obvious inclusions (defects).
Color: D-G = colorless, expensive, only if you have money to burn. H-J = best value. Can go lower in gold metal settings than white metal.
Cut: Better cut ratings let more light into a diamond, making it sparkle more. Very important property, don't skimp here.
Set a budget and minimum cut (Premium). Go J color for gold and I/H for white metals. Go searching for SI1/SI2 clarity diamonds at James Allen. Pick a diamond with small/no inclusions. Choose a ring setting and buy it risk-free (60-day returns).
0.90 Carat Diamonds
If you're interested in the normal metric weight of a 0.90 carat diamond, it is 180 mg.
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Girdle thickness is a diamond property which many people never even think about, but which can critically affect how big the diamond appears to be.
Walk into a diamond shop and wait around, and sooner or later you'll hear someone getting confused or frustrated about the fact that the carat weight of a diamond and the width of the diamond are very different things - most likely they will be confused or frustrated because a diamond with double the carat weight is only a little bit wider.
0.90 carats specifies a certain size of diamond, but what about all the other properties of the diamond? Seasoned diamond buyers make sure to pay attention to all the properties of the diamond when the searching for that perfect stone, in order to make a balanced purchase which isn't just dominated by size considerations.
Ruby and sapphire are two gemstones which are significantly denser than diamond, which is why a one carat ruby or sapphire will generally looks smaller than a one carat diamond, if you've ever been confused in a store. A large diamond which has a huge crack down the middle, or alternatively a big yucky looking inclusion, is not a good look - it looks like someone has gone for size and sacrificed all the other desirable properties of the diamond. Clarity, for example, can be just as important in picking a diamond.
There are three things extremely hard: steel, a diamond, and to know one's self. Benjamin Franklin .