Why one diamond lights up a room and another just sits there.

Because it is the one quality you can see from across a table. Color and clarity differences at the top of the scale take a loupe, white paper and a trained eye to find. A poorly cut stone looks dull to anyone, from anywhere, in any light.

So I spend the budget on cut and on the size you can see first, and on the grades you cannot see without magnification last.

You pay for a diamond by weight, but you look at its width. In the study, a well cut 1.50 carat round measures 7.45 mm across. Cut it too deep and the same 1.50 carats measures 7.07 mm, which is the face of a well cut stone of about 1.28 carats.

The rest of the weight is underneath, below the setting, where nobody will ever see it. And a bottom that steep lets light fall out the sides, so the middle goes dark. You pay more and you get less to look at.

It is tempting, and some sellers lean on it. A shallow stone does face up wider for its weight. The trouble is the bottom of a diamond works like a set of mirrors, and they only send light back to your eye when they sit at the right slope.

Flatten them and the light slips out instead. The stone looks big and glassy, with a washed out pattern and weak sparkle when it moves. Wide and lifeless is not a bargain.

The clarity grade tells you roughly how much is inside the stone. It does not tell you whether you will notice it. A dark crystal under the big flat facet on top sits in the one place your eye always lands, and the facets below can repeat it so one mark reads as three.

Move that same mark out near the edge and a prong can cover it for the life of the ring. Same grade, same price on a listing, and a completely different diamond to live with.

Because crisp or milky is not graded anywhere on a standard report. A faint haze through a stone softens every edge in its pattern, like a window that needs cleaning. It can carry a top cut grade and beautiful letters and still look sleepy.

Stones like that tend to end up sold to people who never see them before they pay. It is one of the first things I check, and the easiest to catch with the stone in front of you.

Because past a certain point you are paying for differences nobody will ever see on a hand. The top grades exist, and they are rare, and they are priced for that rarity. A few steps down, a well chosen stone looks identical face up, set in metal, at arm's length.

Where that point sits depends on the shape, the size and the metal, which is why I would rather look at the stone than give you a rule. The money saved goes into cut and size, where you can see it.

Jewelry store lighting is rows of small, hard spotlights, and it makes every diamond dance, including the weak ones. It is built to do that.

A ring gets worn at a desk, in a car, by a window, at dinner. So I look at a stone in soft daylight and ordinary room light before I call it good, and the photos and video I send you are shot the same way.

Some diamonds glow blue under ultraviolet light. The report lists it as a strength, from none to very strong, and stops there. In most stones you will never see any effect. In a few, strong fluorescence leaves an oily, hazy look in direct sun.

The paper cannot tell you which kind you are looking at. The stone can, in about ten seconds outdoors.

Grading is a trained opinion, and labs are not equally strict. The same stone can earn different letters at different labs. A price that looks like a bargain is sometimes just a generous report.

When I compare quotes for a client, the first thing I check is whose opinion the price is built on.

Because round is the only shape where the angles are standard enough to model honestly. It is also the only shape a lab gives a cut grade to.

Ovals, cushions, pears, radiants, emerald cuts and marquises carry no cut grade at all. Everything you just learned about light coming back or leaking out applies to them twice over, with nobody grading it. For those shapes, looking at the actual stone is the whole job.