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).
Cheap Diamonds For Sale
If you are looking for cheap diamonds you have a number of options, but are really only one option is the best - buying online from a large reputable online diamond merchant.
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If you are hardly unethical, then the cheapest diamonds would probably be conflict diamonds - that is, those that have been obtained under dubious circumstances in probable war zones, or that have been used to fund the purchase of arms.
However I'll assume that you're a decent human being and you're just trying to get the best deal you can out of legitimate diamond sellers.
Consequently, your options are pretty much limited to buying the diamond online. Physical stores - that is, "brick and mortar" stores, have a lot of overheads to cover in terms of renting the store space, advertising, security and paying staff. Their prices are higher to cover these costs.
In contrast, online companies such as James Allen or Blue Nile are able to offer diamonds at significantly cheaper prices because they have much lower operating overheads.
The other advantage of these online companies is that they offer a huge range of diamonds which in effect will help you get a cheap diamond. Because you won't be compromising on getting exactly the type of diamond you want, with the specific diamond characteristics you want, you won't be wasting money on stuff you don't want or need.
A good example of this is when you compare the SI1 and SI2 inclusion diamonds are available at James Allen with much higher clarity diamonds offered by some stores. Because you can look at the photos of the diamond is available from James Allen, you can choose a diamond which is eye clean - that is, with no inclusions that would be visible to the naked eye - and pay a lot less money than if you bought say a VS1 clarity diamond from a store.