HPHT Lab Grown Rough Diamonds
| Technology: HPHT (Temperature Gradient) | Size: Up to 10ct |
| Color: D-H | Clarity: VVS/VS/SI |
| Diamond Type: Type Ib / Type IIa | Crystal Form: Cubo-octahedral |
What Are HPHT Lab Grown Rough Diamonds?
HPHT lab grown rough diamonds are as-grown diamond crystals produced by the High Pressure High Temperature method — the same thermodynamic conditions that form natural diamonds. These uncut, unpolished crystals are the starting material for every lab grown polished diamond: a manufacturer buys rough, plans the cut, and transforms each crystal into faceted gemstones.
HPHT rough is grown using the temperature gradient method: a metal catalyst (iron, nickel, cobalt) dissolves high-purity graphite at ~1,300-1,600°C under 5-6 GPa pressure, then carbon precipitates atom by atom onto a diamond seed crystal at the cooler zone. Growth takes 3-14 days depending on target size. For Type IIa (colorless) rough, a nitrogen getter such as aluminum or titanium is added to prevent nitrogen from entering the crystal lattice.
The characteristic crystal shape of HPHT rough is cubo-octahedral — a well-defined polyhedron with both cubic {100} and octahedral {111} faces. This morphology visually distinguishes HPHT rough from CVD rough (flat plates) and natural rough (rounded, resorbed surfaces). The cubo-octahedral form is also why HPHT rough yields higher polished weight retention — the crystal shape is inherently closer to a round brilliant than a flat plate or irregular natural stone.
Diamond Types: Type Ib vs. Type IIa
Type Ib Rough contains isolated nitrogen atoms (25-200 ppm), giving a yellow to amber body color. It accounts for 70-80% of HPHT production. The yellow tint is removed through post-growth HPHT annealing. Type Ib is the cost-effective choice when the target polished color is G/H or warmer.
Type IIa Rough contains negligible nitrogen (<1 ppm), resulting in colorless as-grown crystals. It requires a nitrogen getter in the growth cell and commands a price premium because it eliminates the decolorization step, making it the preferred starting material for D/E/F color polished diamonds. Type IIa accounts for approximately 20-30% of gem-quality HPHT production.
Product Specifications
| Specification | Details |
|---|---|
| Growth Method | HPHT (High Pressure High Temperature) — temperature gradient method |
| Diamond Type | Type Ib (yellow-tinted as-grown) / Type IIa (colorless, nitrogen-gettered) |
| Crystal Morphology | Cubo-octahedral — well-defined {100} cubic and {111} octahedral faces |
| Size Range | From sub-millimeter to 10ct (approximately 10mm+ for largest crystals) |
| Color (As-Grown) | Type Ib: Light yellow to amber | Type IIa: Colorless to near-colorless (D-H equivalent after treatment) |
| Clarity (As-Grown) | VVS / VS / SI — metal catalyst inclusions are the primary clarity characteristic |
| Crystal Purity | >99.95% carbon — verified by combustion analysis or EDX |
| Thermal Stability | Stable to ~700°C in air, ~1,600°C in inert atmosphere or vacuum |
| Standard Sizes | Sub-1mm, 1-2mm, 2-3mm, 3-4mm, 4-5mm, 5-6mm, 6-8mm, 8mm+ (up to 10ct) — sorted by sieve or caliper |
| Packaging | Parcels sorted by size range and quality grade; labeled with carat weight, count, and grade |
| Lead Time | 7-14 business days for standard size/quality combinations from inventory |
From Rough to Polished: What Manufacturers Need to Know
Weight Yield: A typical HPHT rough crystal yields 25-45% of its carat weight as polished diamond — the rest becomes diamond powder during cutting. Fancy shapes reduce yield by 5-10 percentage points compared to rounds. The single biggest factor in yield is not the rough quality grade — it is the skill of the diamond planner who maps inclusions in 3D and orients the cut to exclude them from the finished stone. A poorly placed inclusion under the table facet can drop a stone from VVS to SI, cutting market value by 40% or more.
Metal Catalyst Inclusions: Microscopic particles of Fe, Ni, or Co trapped during growth are the most common clarity characteristic in HPHT rough. These metallic inclusions are the planner's primary challenge — mapping their position and orienting the proposed cut to exclude them determines polished profitability far more than the initial rough grade.
Quality Assurance
Every parcel of HPHT rough diamonds from Yuda Crystal undergoes the following quality control checks before shipment:
- Size Grading: Sorted by calibrated sieves or digital caliper (4mm+) to confirm every crystal falls within the specified size range
- Crystal Morphology Inspection: 10× magnification examination of form regularity, edge sharpness, and surface characteristics
- Clarity Screening: 10× magnification under diffused lighting to identify metal catalyst inclusions, fractures, and growth defects
- Color Sorting: D65 daylight-equivalent lighting; Type Ib and Type IIa crystals strictly separated
- Carat Weight Verification: Full parcel weighing plus spot-checks of individual crystal weights for larger stones
- Parcel Documentation: Labeled with size range, diamond type, quality grade, carat weight, crystal count, and lot number
Frequently Asked Questions
What is the difference between HPHT rough diamond and natural rough diamond?
Both are real diamonds — chemically (pure carbon), physically (Mohs 10, 3.52 g/cm³), and optically (refractive index 2.42) identical. The differences are: (1) Origin — HPHT rough is grown in days to weeks in a lab; natural rough formed over billions of years in the Earth. (2) Crystal shape — HPHT rough shows sharp cubo-octahedral faces; natural rough has rounded, often etched surfaces. (3) Inclusions — HPHT rough contains metallic catalyst inclusions (Fe, Ni, Co); natural rough contains mineral inclusions (garnet, olivine). (4) Nitrogen type — HPHT rough is predominantly Type Ib (isolated nitrogen); most natural diamonds are Type Ia (aggregated nitrogen). These differences are detectable with spectroscopy but invisible to the naked eye.
How do I choose between Type Ib and Type IIa rough for my production?
Choose Type Ib if your target polished color is G/H or warmer — the yellow tint is removed through standard HPHT annealing, and Type Ib is 15-30% less expensive per carat. Choose Type IIa if your target is D/E/F colorless — negligible nitrogen eliminates the decolorization step and delivers more consistent premium color results. Many manufacturers stock both: Type Ib for commercial-grade, Type IIa for premium lines.
What polished yield should I expect from HPHT rough?
Polished yield typically ranges from 25-45% of rough carat weight: well-formed crystals with high clarity can reach 35-45% for round brilliants, while irregular crystals with visible inclusions may yield 15-25%. Fancy shapes reduce yield by 5-10 percentage points compared to rounds. These ranges assume experienced cutters with proper planning equipment. New operations should budget toward the lower end until their processes are optimized. The single biggest factor in yield is the skill of the diamond planner — mapping inclusions in 3D and orienting the cut accordingly determines profitability far more than the initial crystal appearance.
Can HPHT rough diamonds be used for industrial applications?
Yes. While gem-quality rough with good crystal form and clarity is cut into polished diamonds for jewelry, crystals with irregular shapes or visible inclusions are used directly in industrial applications: SPDT cutting tools, diamond dressing tools, wire drawing dies, hardness indenters, and diamond anvil cells. Crystals unsuitable for jewelry still have excellent hardness and thermal conductivity for industrial use.
Please do not hesitate to contact us for current rough diamond pricing, available size and quality combinations, or to discuss custom rough diamond specifications for your cutting and polishing operations.
