Prongs show the diamond off; a bezel protects it. Four prongs leave the most stone exposed and look light and open. Six prongs hold a round more securely and keep its outline circular. A bezel wraps the edge in a smooth rim of metal so nothing snags on a scarf or sweater, at the cost of a little light.
On a necklace that turns and catches light as you move, that small loss matters less than it would on a ring, so a bezel is a sound pick for a piece you never take off. Prongs suit you if maximum sparkle is the goal and the pendant will mostly come out for evenings.
Size comes down to proportion and budget. As a guide for rounds, a half carat measures about 5.2 mm across, one carat about 6.4 mm and one and a half carats about 7.4 mm, so each step is visible without being dramatic. At necklace distance an eye clean SI1 or VS2 looks as clean as a higher grade, which frees money for size or cut. Weights a little under a round mark cost noticeably less with no visible difference. Finished pieces such as 1 carat solitaire necklaces are a useful way to judge scale on the neck.
With solitaire diamond necklace settings, the chain is half the look. A fine cable or box chain keeps every eye on the diamond. Rope, Franco and Cuban chains add texture and weight, and a paperclip chain gives a modern, open feel. Match the chain's weight to the diamond: a delicate chain suits a small diamond, while a heavy one can overpower it. Length is personal, but a pendant that rests near the collarbone shows well with most necklines. Check the clasp too, since a sturdy one is far easier to fasten on your own than a tiny one.
For a solitaire, start from the stone. The diamond is the whole design, so its shape and size decide which head fits and how the loop at the top will look beside it. Browse round cut diamonds for the classic choice, or a pear or oval for something that hangs with more movement. Shapes with points, such as a pear, need a V prong at the tip, just as they would in a ring. The build your own pendant page walks through the steps if you have not chosen a stone yet.