A halo diamond pendant surrounds the center with a border of smaller stones, so from a normal distance the eye takes in one larger, brighter shape. That matters more on a necklace than on a ring. A pendant is usually seen from across a table or a room, where a small diamond on its own can get lost against skin and fabric; the halo gives it width and a ring of light that holds up at that distance.
It also changes the mood. A solitaire reads minimal and modern, while a framed stone feels more glamorous and dressed up, and it moves easily from evenings to everyday wear.
Choose the shape first, because the frame is built to follow the outline of the stone: a round in a round halo, an oval or pear in a border that traces its curve. Elongated shapes already look larger for their weight, and inside a frame of small diamonds the two effects add up.
Spend on cut so the center holds its own against the sparkle around it, and keep clarity eye clean; at necklace distance, finer grades are hard to tell apart. Because the halo adds width, a center a little below a round weight looks generous and costs noticeably less. If you want the most size for the budget, look at lab grown diamonds, which are the same material as mined stones and cost far less at the same size and grade.
This style looks finished alongside studs of the same character. Stud settings come in halo styles too, and you can create your own earrings from a matched pair of loose diamonds chosen to align closely in size, color and clarity. For the chain, something fine lets the halo be the focus, and a length that rests near the collarbone keeps it on show with most necklines. If you would rather keep it to one diamond, compare solitaire pendant settings.
If the setting you like comes in more than one metal, pick it by how you want the frame to read. White gold and platinum blend with the small diamonds, so the center and its border look like one bright disc; that suits a colorless or high near colorless stone, since white metal shows warmth. Yellow and rose gold outline each little stone in warm color and hide a slightly warmer center well, which can free budget for a larger stone. Match the chain to the setting so the metal looks continuous from bail to clasp.