Half the advice you read about the 4Cs is recycled marketing. Here are three myths worth unlearning before you compare prices.
Myth 1: "D colour is the best"
D is the top of the colourless scale, but the jump from D to G is nearly invisible on a white gold or platinum setting, and the price difference is large. The jump from G to J on a yellow or rose gold setting is also nearly invisible. You are paying a premium for something you cannot see at arm's length.
Myth 2: "SI1 is always eye-clean"
Not true. "Eye-clean" means you cannot see inclusions without a loupe. Some SI1 stones are eye-clean; some are not. The position of the inclusion matters — one under a prong is invisible; one in the middle of the table is obvious. Always ask the seller for a video or loupe view.
Myth 3: "Bigger carat always looks bigger"
Carat is weight, not face-up size. A shallow-cut 1ct diamond can have a tiny table and look smaller than a well-cut 0.8ct. Elongated shapes (oval, pear, marquise) look larger per carat than rounds because more of the weight sits on the surface.
The takeaway
Buy the cut first. Then look at the actual stone, not just the grade. If you cannot see the flaw, it does not matter what the certificate says.