It makes your diamond look bigger and brighter by surrounding it with a border of small diamonds. From a few feet away, the eye reads the stone and its border as one larger shape, and the tiny diamonds throw extra light even when the main diamond is modest. That is the main reason people pick halo style engagement rings: they stretch the visual size of whatever budget you bring.
Halos also give a slim finger real presence without a heavy band, so you can keep the shank delicate and still wear a ring that gets noticed.
The halo follows the outline of the stone it holds, so settle the shape before you fall for a design. A round sits in a round border and an oval in an oval one, while a pear or cushion gets a frame that traces its curves. Elongated shapes such as oval and pear already look larger per carat, and inside a diamond border that effect stacks.
Lab grown diamonds deserve a look here. They are the same material as mined stones, they cost far less at the same size and grade, and that saving can go toward a better cut. Browse lab grown diamonds by shape to see what suits the setting you like.
Because the border adds visible width, you can choose a stone a step smaller than you would for a solitaire and still end up with a generous looking ring. Put cut first, since your diamond has to hold its own against all the sparkle around it. Clarity can stop at eye clean, and a near colorless grade in the G to J range keeps the middle stone from looking tinted beside bright accents. A diamond just shy of a round weight, say 1.90 to 1.99 carats instead of two, costs noticeably less and looks identical once framed.
Think about the width of the frame first: a single slim row of small stones keeps the look light, while a wider border makes a bolder statement. Plan the wedding band early too. A straight band can leave a small gap under a wide halo, while a curved or notched band follows its outline so the pair sits flush. If you want the extra sparkle with a cleaner top view, compare hidden halo settings, which tuck the small diamonds underneath the main stone. For finished ideas, see halo engagement rings for women.