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).
Actual Diamond Size of a 1.65 Carat Diamond
If you're thinking about how large a 1.65 carat diamond appears on a hand, take a look at this picture to give you an approximate idea. A typical round diamond with a 1.65 carat weight has a diameter of 7.7 mm; bear in mind that this can vary slightly depending on the cut.
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Be careful of reading too much into just a picture - it can't be completely representative of all finger and hand types - you are best advised to get a bit of an idea in store with an actual ring. Do not forget to take into account not just the size of your hand as a whole, but in addition if you possess long and thin fingers or short and pudgy ones. The "first impressions" size of a diamond is influenced by what it is shown against - her hand - a small hand will make the diamond appear proportionately larger and vice versa.