With a 10 power diamond loupe, a gauge, the proper light and some experience you may be able to approximate the size and quality of diamond dealers.
While the weight of diamond necklace is important, it doesnt necessarily provide the "perceived size" of fancy diamonds. A shallow cut diamond will have a greater diameter, and will therefore look larger.
We shall probably never know when the first wedding bands was discovered, but we do know that, from ancient times until the eighteenth century, all the worlds fancy diamonds came from India.