Shape is the first thing anyone notices and the last thing the grading report cares about, which makes it the most personal decision on the ring. It is also one of the few places where taste can save you real money.
The incumbent
The round brilliant holds most of the market because it returns the most light and flatters the most hands, and it charges handsomely for both. More of the rough crystal is sacrificed to cut one, and demand does the rest. Round is never a mistake. It is merely never a bargain.
The challengers
Ovals stretch the finger and carry their weight across more visible area, so they tend to look larger per carat than rounds, which explains their long run of popularity. Pears and marquises push that lengthening effect further and bring a little drama with it. Emerald and asscher cuts trade sparkle for glassy, architectural flashes, and they confess clarity flaws more readily, so the report matters more there. Cushions and radiants split the difference, soft-cornered and lively. Princess cuts deliver brilliance at a discount, with corners that sensible settings protect.
Fancy shapes often price noticeably below rounds of equal weight. Taste, for once, is subsidized.
Choosing without a degree
Her existing jewelry and saved photos will usually declare a silhouette long before any showroom does. When the evidence is mixed, oval and round are the two shapes nobody has ever returned on principle.
The short of it: round for certainty, oval for size per dollar, the angular cuts for character, and her saved photos outrank the market's opinion.