All over the world, India is deemed to be the region where diamonds are found at cheaper rates. This certainly has many other reasons as well.

A sweet spot in the compromise between size and price, diamonds around the 0.70-ct mark make great engagement ring stones. A diamond budget of about $2,000 can get you a fine diamond at this weight.

In that article, three Ohio diamonds, all found in the last century, were described: a stone found in Cuyahoga County, south of Cleveland, about 1870; an occurrence in Hamilton County about 1880; and a discovery in 1899 in Clermont County.

so, depending on your buying range. but India is the cheapest followed up by China, Dubai, Thailand, and Belgium. they are the cheapest because most of the world's diamonds are cut there. so you do not have to pay any markup due to shopping or retailer markup.