How we choose your diamond
Every diamond you see in our ring builder has already been checked before it reaches the list. This page explains what we look at, and why most of what our supplier offers never gets in front of you.
We only show CVD stones
Lab grown diamonds are made two ways. CVD grows the crystal from a carbon rich gas at low pressure. HPHT grows it under extreme heat and pressure, closer to the conditions that form diamonds underground.
Both produce real diamonds, chemically and optically identical to mined ones. We buy CVD only, across everything we sell.
HPHT stones more often carry a faint colour cast or internal strain, and a proportion of them are treated after growth to improve their colour. Neither is a defect, and plenty of jewellers sell them happily. We would rather hand you a stone with a simpler history.
There is a practical reason too. Some HPHT stones contain traces of boron, which makes them conduct electricity. On the combination testers many jewellers use, that can flag a genuine diamond as moissanite or return an unclear reading. Some HPHT stones also carry tiny metallic inclusions left over from the growth process, which means a strong magnet can move them. Neither changes what the stone is, and a proper thermal tester still reads it as diamond, but we would rather you never have that conversation with a valuer or an insurer.
This rules out a great deal of stock. Looking for a one carat round in our colour and clarity range on one recent day, we went through two hundred stones to find thirty we were willing to show. One hundred and seventy of them were HPHT.
Cut, polish and symmetry
Every stone we list is graded Excellent for polish and Excellent for symmetry. There is no second tier here and no exceptions.
For round brilliants we also require an Excellent cut grade. Fancy shapes are different. IGI does not issue a cut grade for ovals, pears, emeralds or the rest, because the trade has never agreed a single standard for how those should be proportioned. So for fancy shapes we check the proportions ourselves, shape by shape.
The proportions we look for
A well made diamond is mostly a question of geometry. Light enters through the table, bounces off the pavilion facets and comes back to your eye. Get the angles wrong and the light leaves through the bottom of the stone instead, which is why two diamonds of the same weight, colour and clarity can look quite different on the hand.
These are the ranges we work to:
| Shape | Length to width ratio | Table | Total depth |
|---|---|---|---|
| Round | 0.99 to 1.00 | 57 to 59% | 60 to 63% |
| Oval | 1.40 to 1.48 | 57 to 63% | 60 to 66% |
| Pear | 1.55 to 1.65 | 55 to 61% | 58 to 62% |
| Emerald | 1.40 to 1.51 | 62 to 67% | 62 to 67% |
| Radiant | 1.25 to 1.45 | 62 to 67% | 62 to 67% |
| Cushion | 1.00 to 1.40 | 64 to 74% | 62 to 67% |
| Princess | 1.00 to 1.03 | 67 to 72% | 64 to 75% |
| Marquise | 1.85 to 2.15 | 55 to 62% | 58 to 64% |
| Heart | 0.95 to 1.02 | 53 to 63% | 58 to 64% |
Ratio matters most, because it is the thing you actually see. A pear at 1.45 looks stubby. The same pear at 1.60 looks like a pear.
Shade
Some lab grown diamonds carry a faint grey or brown nuance that sits outside the normal colour grade. A stone can be graded D for colour and still read slightly dull because of it. We exclude anything with a recorded shade.
Colour and clarity
We list D, E and F for colour, which are the three colourless grades. For clarity we list FL through to VVS2. Every stone comes with an IGI certificate, and the report number is on the stone's card in the builder so you can look it up yourself before you commit to anything.
What the Luma Recommends badge means
When you browse diamonds, some carry a Luma Recommends badge. Those are the stones whose proportions sit inside the ranges in the table above for that shape.
Everything we list has already passed the checks on this page. The badge is the next level up, and it is where we would put our own money.
We may come back to you with something better
Our team looks at every order before it reaches the bench. Between us we have spent our working lives in Hatton Garden looking at stones, and a certificate only tells you so much.
If we think there is a better stone available for what you are spending, or for very little more, we will contact you and say so. Sometimes that means a slightly smaller stone that performs better. Sometimes it is the same stone from a different parcel. You are free to say no and we will make the ring exactly as you ordered it.
We are not going to pretend this happens on every order. It happens when it is worth telling you about.
Questions we get asked
Are lab grown diamonds real diamonds?
Yes. Same carbon, same crystal structure, same hardness, same sparkle. A gemmologist needs specialist equipment to tell one from a mined stone, and the difference is where it grew, not what it is.
What is the difference between CVD and HPHT?
Only the growth method. CVD builds the crystal layer by layer from a carbon rich gas. HPHT grows it under enormous heat and pressure using a metal catalyst. We list CVD only, for the reasons above.
Do you sell HPHT stones at all?
No. Not in the ring builder, not in our finished jewellery, and not in the accent stones set into our rings. We have never bought them and we do not intend to.
If you have been quoted an HPHT stone elsewhere and want a second opinion, send us the certificate and we will read it with you.
Why do some stones have a Luma Recommends badge and others do not?
Every stone we list has passed the checks on this page. The badge marks the ones whose proportions also sit inside our preferred range for that shape. An unbadged stone is not a bad stone. It simply sits a little outside where we would aim.
Can I see the certificate before I order?
The IGI report number sits on every stone in the builder. You can check it on IGI's own site before you spend anything. Start with the engagement ring collection and pick a setting to open the builder.
I know the shape I want. Where do I start?
Go straight to the shape: round, oval, pear, emerald, trilogy, halo or solitaire. Choose a setting and the diamond search opens inside the product page.
What if I want something that is not on the site?
That is what Luma Bespoke is for. Bring a sketch, a photograph or a rough idea and we will design around it. You can also work it up yourself in the Design Studio.
Can I see the stone before I commit?
Yes. Book a slot at the studio in Hatton Place and we will have the options out on the bench when you arrive. Most people find that ten minutes with two stones side by side settles a decision that a week of reading could not.
Something else?
Our FAQ page covers timings, sizing, resizing and returns.