CVD vs HPHT Lab Grown Diamonds: What's the Difference?
CVD and HPHT are the two methods used to grow lab diamonds. Both produce genuine diamonds, chemically identical to mined diamonds, IGI certified, and Mohs 10 hardness. The difference is the process: CVD grows diamonds atom by atom in a low-pressure vacuum chamber over 4–8 weeks; HPHT replicates the earth's mantle, subjecting carbon to extreme heat and pressure over days to weeks.
For a diamond necklace buyer, the honest answer is: the growth method is the least important factor in how your diamond looks and wears. The certified 4Cs on your IGI certificate matter far more. This guide explains how each method works, how they differ in the finished stone, and exactly what to look for when you see "CVD" or "HPHT" on a diamond certificate.
How Lab Grown Diamonds Are Made: The Starting Point
Both CVD and HPHT begin with a diamond seed, a thin slice of existing diamond, typically a few millimetres across. This seed acts as a foundation on which new carbon atoms attach and crystallise. The chemistry is the same goal regardless of method: create the conditions that cause pure carbon to form the cubic crystal lattice structure that defines a diamond.
What differs between CVD and HPHT is how those conditions are created.
CVD, Chemical Vapour Deposition
CVD is the more widely used method for gem-quality lab grown diamonds today. It was developed in the 1980s and refined significantly through the 1990s and 2000s. The process is inspired, somewhat unexpectedly, by how diamonds form in interstellar space, in cosmic gas clouds far from any planet.
How CVD Works
A diamond seed is placed inside a sealed vacuum chamber. The chamber is then filled with a carbon-rich gas mixture, typically methane (CH₄) combined with hydrogen. Microwave energy is applied to the chamber, ionising the gas into plasma at temperatures of approximately 700–1,300°C.
In this plasma state, the molecular bonds in the carbon-rich gas break apart, releasing individual carbon atoms. These atoms rain down onto the diamond seed and bond to its surface, building the diamond upward one atomic layer at a time, much like sediment depositing on a riverbed, but atom by atom.
Growth direction: CVD diamonds grow in a single direction (upward), forming a tabular, cubic shape.
Growth time: A gem-quality CVD diamond typically takes 4–8 weeks to grow, depending on the target carat weight and quality.
Pressure: Near-atmospheric, the CVD chamber operates at low pressure compared to HPHT.
What CVD Diamonds Look Like
Because the CVD process operates in a precisely controlled environment, CVD diamonds frequently achieve high purity. Many CVD diamonds qualify as Type IIA, the highest purity classification in diamond science, meaning they contain almost no nitrogen or boron impurities. Only approximately 2% of naturally mined diamonds are Type IIA; CVD diamonds reach this standard more consistently.
CVD diamonds can occasionally exhibit:
- Faint growth lines or graining, visible only under laboratory instruments, not to the naked eye
- A slight brownish tint in their raw, unfinished state,a result of vacancy defects accumulated during growth. This is typically eliminated through standard cutting and polishing, but some CVD diamonds undergo additional post-growth treatment to address it (more on this below)
- Blue nuance — a faint blue or grey cast caused by trace boron in some CVD growth environments; disclosed on the IGI certificate when present
All of these characteristics are assessed and graded during IGI certification. A CVD diamond graded D colour is colourless — the brownish raw tint has been resolved in the finished, certified stone.
HPHT — High Pressure High Temperature
HPHT was the first method used to grow diamonds in a laboratory. It was developed by General Electric in the 1950s, initially producing small, heavily included industrial diamonds. The technology has improved significantly since then, and HPHT now produces gem-quality diamonds used in fine jewellery.
How HPHT Works
A diamond seed is placed inside a sealed press along with a carbon source — typically graphite — and a metal catalyst, usually a mixture of iron, nickel, and cobalt. The assembly is then subjected to conditions that replicate the earth's mantle:
- Temperature: 1,300–1,600°C
- Pressure: 5–6 GPa (gigapascals) — equivalent to approximately 870,000 pounds per square inch, or the pressure found roughly 150 kilometres below the earth's surface
At these extremes, the metal catalyst dissolves and the carbon source begins to crystallise onto the diamond seed, growing outward in 14 directions simultaneously to form a cuboctahedron shape — the same geometry as the crystals produced in the earth's mantle.
Growth time: HPHT diamonds typically grow in a few days to several weeks, depending on the target size and the specific press configuration.
What HPHT Diamonds Look Like
HPHT diamonds have distinct growth characteristics that differ from CVD:
- Metallic inclusions — traces of the iron, nickel, or cobalt catalysts can become embedded in the diamond during growth. These inclusions are assessed and graded by IGI and reflected in the clarity grade. They are one of the markers gemologists use under laboratory instruments to identify HPHT-grown stones
- Colour tendencies — HPHT diamonds can exhibit a faint yellow or brownish tint from nitrogen present in the growth environment, or occasionally a blue/grey nuance from boron. Top-grade colourless HPHT diamonds (D–F) have been refined out of these tendencies
- Cuboctahedron growth structure — the 14-direction growth produces a stone shape particularly well suited to round brilliant cuts
HPHT is also used as a post-growth treatment on some diamonds — applied after growth to remove colour from CVD diamonds that emerged with a brownish tint. When this has been done, the IGI certificate reflects it (the stone will not carry an "As Grown" notation).
CVD vs HPHT — Side by Side
| CVD | HPHT | |
|---|---|---|
| Full name | Chemical Vapour Deposition | High Pressure High Temperature |
| Developed | 1980s | 1950s |
| Temperature | 700–1,300°C | 1,300–1,600°C |
| Pressure | Low (near-atmospheric) | 5–6 GPa (~870,000 PSI) |
| Growth time | 4–8 weeks | Days to several weeks |
| Growth directions | 1 (tabular / cubic shape) | 14 (cuboctahedron shape) |
| Raw crystal shape | Tabular plate | Cuboctahedron |
| Inclusions | Mineral inclusions possible; no metallic inclusions | Metallic inclusions possible (iron, nickel, cobalt) |
| Type IIA achievability | Very common | Less common but achievable |
| Colour tendency (raw) | Occasional brown tint | Occasional yellow/brown or blue nuance |
| Post-growth treatment | Sometimes (to remove brown tint) | Sometimes (to improve colour) |
| Suited to | Fancy cuts (oval, pear, emerald, cushion) | Round brilliant |
| IGI certificate notation | "CVD" | "HPHT" |
| Quality potential | Top grades achievable | Top grades achievable |
How CVD and HPHT Diamonds Differ in the Finished Stone
When a diamond has been cut, polished, and independently certified by IGI, the growth method leaves very little practical difference in what you hold, wear, or see.
Appearance: Two diamonds graded D/Excellent/VS1 — one CVD, one HPHT — are visually indistinguishable. Both will have Mohs hardness 10, refractive index 2.417, and dispersion 0.044. Neither a trained jeweller with a loupe nor a standard diamond tester can tell them apart.
Inclusions: CVD diamonds may have mineral inclusions or faint growth lines; HPHT diamonds may have metallic inclusions. In both cases, these are assessed at IGI and reflected in the clarity grade. A VS1 or VS2 grade from either method means the inclusions are invisible to the naked eye.
Colour: Both methods can produce top colour grades (D, E, F). Historical tendencies — CVD toward slight brown raw tints, HPHT toward slight yellow/brown — have been significantly mitigated by process improvements and apply to raw, unfinished stones. A certified D colour grade from either method is colourless.
What can differ under laboratory instruments: Specialised equipment used by gemological laboratories — not available in retail settings — can detect growth patterns, internal strain, and inclusion types unique to each method. This is how IGI identifies and discloses the growth type on a certificate. It has no bearing on the stone's wearability or visual beauty.
What Your IGI Certificate Tells You
When you receive an IGI certificate with a Bellari diamond necklace, the certificate specifies the growth method. Here is what to look for:
"Grown by CVD" or "CVD" in the origin/comments section — confirms the stone was grown using Chemical Vapour Deposition.
"Grown by HPHT" or "HPHT" in the origin/comments section — confirms the stone was grown using High Pressure High Temperature.
"As Grown" — this notation indicates that the diamond's colour and clarity are its natural result from the growth process; no post-growth treatments have been applied. This is the highest transparency standard and is considered preferable from a provenance perspective.
Absence of "As Grown" — does not necessarily indicate a treated stone, but it is worth confirming with the jeweller. Post-growth HPHT treatment on CVD stones is a legitimate, industry-disclosed process — it simply means an additional step was applied to improve the stone's colour, which the IGI certificate reflects.
You can verify all certificate details — including growth method and any treatment notations — at igi.org using your report number.
Does CVD vs HPHT Matter for a Diamond Necklace?
No — not for the stone's appearance or performance as a necklace.
This is the question most buyers are really asking, and the honest answer is that the growth method is the last thing to consider when choosing a diamond necklace.
Here is a practical way to think about it: if you compared two solitaire necklaces side by side — one with a CVD diamond graded D/VS1/Excellent and one with an HPHT diamond graded D/VS1/Excellent — you would not be able to identify which was which by looking at them. Neither would anyone who saw you wearing the necklace. A gemologist in a laboratory with specialised instruments could identify the growth method, but that test is never performed outside of a gemological laboratory.
What determines how beautiful your diamond necklace looks is the same for CVD and HPHT:
Cut grade — the single most important factor. An Excellent cut maximises light return and gives the diamond its brilliance regardless of growth method.
Colour grade — how white and bright the stone appears. D, E, F are colourless. In a white gold setting, stick to D–F. In a yellow gold setting, G or H works beautifully.
Clarity grade — whether inclusions affect appearance. For a solitaire necklace worn at chest height, VS1 or VS2 is eye-clean and visually perfect.
Carat weight — the physical size of the stone.
These four factors, documented on your IGI certificate, determine how your necklace looks. The CVD or HPHT notation below them is technical provenance information — useful to know, but not a quality indicator.
Which Is Better: CVD or HPHT?
Neither is better as a category. Both methods can produce exceptional diamonds. The diamond industry's tendency to debate CVD vs HPHT often reflects brand bias — sellers who source one method tend to position it as superior. The objective position, supported by the IGI grading system, is that the finished stone's 4Cs determine quality, not its production method.
Where differences do legitimately exist:
CVD is more commonly used for fancy-cut diamonds (oval, pear, emerald, cushion) because the tabular growth shape lends itself to these silhouettes. HPHT's cuboctahedron growth shape is particularly well-matched to round brilliants.
CVD achieves Type IIA classification more frequently, which is a purity benchmark — though HPHT stones can also reach Type IIA.
HPHT has a longer history and has been used in industrial applications since the 1950s; CVD has overtaken it for gem-quality fine jewellery production since the 2000s.
For a buyer choosing a solitaire diamond necklace, these distinctions are not decision-making criteria. Choose by IGI-certified 4Cs, not by growth method.
Post-Growth Treatment: What It Means and Why It's Disclosed
Some lab grown diamonds undergo post-growth treatment — an additional step applied after the stone has been grown but before it is cut and certified. The most common treatment is applying HPHT to CVD diamonds to remove a residual brown tint that developed during growth.
Is post-growth treatment a problem? No — it is a legitimate, disclosed process in lab diamond production, similar to heat treatment in other gemstones (sapphires, for example, are routinely heat-treated and this is standard practice in the industry). The IGI certificate discloses whether treatment has been applied.
How to check: Look for the "As Grown" notation on your IGI certificate. If present, the stone has not been post-treated. If absent, ask your jeweller whether the stone has been treated and verify against the IGI report details.
At Bellari, the growth method and treatment status are fully disclosed on every IGI certificate that accompanies your necklace.
Bellari's Lab Grown Diamond Solitaire Necklace Collection
Every diamond in Bellari's solitaire necklace collection is independently IGI certified, with the growth method documented on the certificate. Stones are selected for their certified 4Cs — Excellent cut, D–H colour, and VS1 or VS2 clarity — in four cuts, each available in 18K yellow gold and 18K white gold.
Round Brilliant
The round brilliant cut is optimised for maximum light return with 58 precisely angled facets. It is the most popular diamond cut globally and the standard against which all other cuts are measured for brilliance.
- Round Brilliant Diamond Necklace — 18K Yellow Gold
- Round Brilliant Diamond Necklace — 18K White Gold
Oval Cut
The oval elongates on the neckline and faces up larger than its carat weight. It retains the brilliance of a round cut while offering a softer, more distinctive silhouette. CVD growth lends itself particularly well to the oval's elongated proportions.
Pear Cut
The pear — or teardrop — combines the pointed brilliance of a round cut with a tapering tip that orients downward in a pendant setting. One of the most elegant shapes for a solitaire necklace.
Emerald Cut
The emerald cut's step facets produce a hall-of-mirrors effect rather than traditional sparkle — architectural, understated, and visually deep. Look for VS1 or VVS2 clarity in an emerald cut, as the open table reveals the interior of the stone more directly than brilliant cuts.
For everything you need to know about choosing between these four cuts and two metal options, read our Lab Grown Diamond Solitaire Necklace Guide.
To understand IGI certification and which grades to look for, read What Does IGI Certified Mean?
Final thoughts,
CVD and HPHT are the two processes used to grow lab diamonds. CVD builds diamonds atom by atom in a vacuum chamber at 700–1,300°C over 4–8 weeks. HPHT mimics the earth's mantle at 1,300–1,600°C and 5–6 GPa of pressure over days to weeks. Both produce genuine, IGI certified diamonds identical in hardness, refractive index, and carbon crystal structure to mined diamonds.
The growth method is disclosed on every IGI certificate and can be verified at igi.org. For a diamond necklace buyer, it is the least important item on that certificate. What determines the beauty of your necklace is the certified 4Cs — particularly cut grade (Excellent), colour (D–H by metal type), and clarity (VS1 or VS2 for brilliant cuts, VS1 or VVS2 for emerald cuts).
If you are new to lab grown diamonds and want to understand the full picture before choosing a necklace, start with Are Lab Grown Diamonds Real? — the foundational guide for Australian buyers.
Frequently Asked Questions
What is the difference between CVD and HPHT lab grown diamonds? CVD (Chemical Vapour Deposition) grows diamonds atom by atom in a vacuum chamber using carbon-rich gas at 700–1,300°C, taking 4–8 weeks per stone. HPHT (High Pressure High Temperature) mimics the earth's mantle at 1,300–1,600°C and 5–6 GPa, growing diamonds in days to weeks. Both produce genuine, IGI certified diamonds identical to mined diamonds in hardness and composition.
Which is better: CVD or HPHT lab grown diamonds? Neither is inherently better. Both can produce top IGI-graded diamonds. The 4Cs on the IGI certificate — cut, colour, clarity, carat — matter far more than the growth method. A D/VS1/Excellent CVD diamond and a D/VS1/Excellent HPHT diamond are visually indistinguishable when worn.
Can you tell the difference between a CVD and HPHT diamond? No — not with the naked eye, a loupe, or a standard diamond tester. Both have identical hardness (Mohs 10), refractive index (2.417), and carbon structure. Specialised laboratory instruments can identify growth patterns, but these are not available in retail settings.
Does CVD vs HPHT matter when buying a diamond necklace? No. The growth method is the least important factor for a necklace buyer. What determines how the diamond looks is the certified IGI grades — cut (Excellent), colour (D–H), and clarity (VS1 or VS2). Two diamonds of the same IGI grade — one CVD, one HPHT — will look identical worn as a solitaire necklace.
What does CVD mean on a diamond certificate? CVD on an IGI or GIA certificate means the diamond was grown using Chemical Vapour Deposition — carbon atoms deposited onto a diamond seed in a vacuum plasma chamber. It confirms laboratory origin and growth method but does not indicate quality, which is determined by the 4Cs grades on the same certificate.
What does HPHT mean on a diamond certificate? HPHT on an IGI or GIA certificate means the diamond was grown using High Pressure High Temperature — mimicking the earth's mantle at 1,300–1,600°C and 5–6 GPa. Like CVD, it confirms laboratory origin and does not indicate quality.
Do CVD diamonds have inclusions? CVD diamonds can have mineral inclusions and occasional growth lines from the layer-by-layer deposition process, but no metallic inclusions. All inclusions are assessed at IGI and reflected in the clarity grade. A VS1 or VS2 CVD diamond is eye-clean regardless of any microscopic growth characteristics.
Do HPHT diamonds have inclusions? HPHT diamonds can contain metallic inclusions — traces of the iron, nickel, or cobalt growth catalysts — and can show a faint yellow or brownish tint from nitrogen. All characteristics are graded by IGI. A top-grade HPHT diamond is colourless and eye-clean.
What is an 'As Grown' lab grown diamond? "As Grown" on an IGI certificate means the diamond's colour and clarity are the natural result of the growth process — no post-growth treatments were applied. Some lab diamonds undergo additional HPHT treatment after CVD growth to improve colour. The IGI certificate discloses whether a stone is "As Grown" or has been treated.
Are CVD diamonds Type IIA? CVD diamonds frequently achieve Type IIA classification — the highest purity level, with almost no nitrogen or boron impurities. Only ~2% of mined diamonds are Type IIA naturally. CVD growth achieves this standard more consistently, producing exceptionally transparent, bright stones.
Does Bellari sell CVD or HPHT lab grown diamonds? Bellari's lab grown diamond solitaire necklaces are independently IGI certified, with the growth method disclosed on each certificate. Stones are selected by their certified 4Cs quality. The growth method is fully documented and verifiable at igi.org using the certificate report number.
