In laboratory-grown diamonds, price per carat at the top grades peaks around five carats and falls above it — a 10.02ct D/VVS2 listed at $735 per carat against a 5.01ct D/VVS2 at $1,136 on September 18, 2026 — so the exponential size premium that governs mined diamonds does not exist here.

That runs against everything the natural diamond market has taught buyers, and it changes what a fair price for a large stone looks like. A buyer who expects a 10-carat stone to cost more per carat than a 5-carat can be charged for a premium that the market itself doesn’t carry.

This Operia guide shows the dated data, where the premium went instead, what sits between a stone’s cost and its price, and how to read a comparison with natural diamonds.

What do large lab-grown diamonds cost?

On September 18, 2026, we checked D-color listings on one online marketplace. The per-carat figure is the listed price divided by the carat weight.

At commercial grades the curve is nearly flat. On the same marketplace and the same day, a 1.00-carat F/VS1 was listed at $385 per carat, a 5.00-carat G/SI1 at $490 and a 10.54-carat G/SI1 at $545: barely more per carat across a more than tenfold jump in size.

How these figures were collected

What the averages leave out

A live-inventory price index puts the average 5-carat laboratory-grown diamond at about $2,336, or $467 per carat, across 100,478 listings. It is a real number, but it averages every color, clarity and cut on the market, including every badly cut stone. Measured against it, any well-cut top-grade stone looks expensive. Measured against stones of the same size and grades, checked on the same day, it may not be.

Why doesn’t a bigger stone cost more per carat?

In mined diamonds, price per carat climbs steeply with size, because large crystals are geologically rare: each step up in weight raises the per-carat price as well as the total. Laboratory-grown diamonds have no such curve. Their prices follow production cost and competition, and both have been pushing prices down.

Edahn Golan Diamond Research’s wholesale data show laboratory-grown prices down more than 97 percent between mid-2018 and mid-2026. The fall has been steepest at the large end. In the second quarter of 2026, wholesale prices fell 13 percent year on year overall, while 1-carat rounds rose 1 percent and stones of 2 carats and above fell 20 percent.

Buyers have responded by buying bigger rather than spending less: retail unit sales rose 24 percent while consumer spending stayed flat, as the same budgets turned into larger stones.

Retail prices show the same inversion. In September 2026, one national retailer listed a 2.03-carat D/VVS1 at $2,581 per carat, more than its own 5.10-carat stone at $1,241 per carat.

Where did the premium go?

Price per carat still varies widely, but by quality rather than size.

These are the differences worth paying for, and they are exactly what a blended average hides.

What are you paying for above the stone?

Edahn Golan Diamond Research puts the typical retail markup on a loose laboratory-grown diamond at 250 to 300 percent, an average gross margin of about 73 percent. In other words, the retail price is typically three and a half to four times what the retailer paid for the stone.

It shows in listings. In September 2026, one specialist online retailer listed a 5.03-carat D/VVS2 at $2,674 per carat, about $13,450 in total. That is about 2.4 times the marketplace’s per-carat price for its 5.01-carat D/VVS2, and more in total than the marketplace’s 10.02-carat stone of the same grades.

The difference pays for the retailer: service, returns, showrooms, marketing and margin. Those can be worth paying for. They are not properties of the stone, and a buyer should be able to see them as what they are.

How do lab-grown and natural prices compare?

Two caveats come first. These pairs are not grade-matched: we could find no D/IF natural diamond at retail at 10 carats or larger, so the natural stones below are the closest available. And both markets are moving, lab-grown prices downward and natural large-stone prices on their own path, so any ratio drawn from these figures goes out of date within months. We show the prices, dated, and leave the arithmetic to you.

Retail listings, checked September 2026. Not grade-matched.

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