How Crossed Polarizers Expose Natural "Tatami" Strain vs. Lab-Grown Diamonds—And Why Every Diamond Needs a Supersonic Kimberlite Rocket
Direct Deep-Mantle Mineralogy, Photoelastic Strain & 2026 IMA Inclusion Summary: Two of the most common myths in geology are that "diamonds come from compressed coal" and that "you need a $20,000 laboratory spectrometer to tell a natural diamond from a lab-grown diamond." In reality, natural diamonds crystallized **1 to 3.5 billion years ago** (hundreds of millions of years before the first land plants or coal existed!) at depths of **150 to 750 kilometers** from primordial mantle carbon. Because those crystals spent eons being squeezed by tectonic mantle convection at 1,200°C, plastic dislocations slipped along their octahedral `{111}` planes—leaving a permanent cross-hatched birefringence shadow called **"Tatami" Strain** that pops right into view between two inexpensive **90° crossed polarizing filters**! By contrast, **HPHT (High-Pressure High-Temperature) lab-grown diamonds** crystallize in days inside a static press and show **zero tatami strain** (plus metallic `Fe-Ni-Co` flux inclusions that jump to an N52 magnet!), while **CVD (Chemical Vapor Deposition) lab diamonds** show parallel columnar strain lines. And why didn't natural diamonds turn into black **Graphite pencil lead** on their way up from 200–660 km depth? Only because `CO2`-charged **Kimberlite magma** rockets upward at **30 to 300 km/h**, quenching the diamond before `sp3` bonds can unzip—and trapping super-deep treasures like **660 km hydrous Ringwoodite** and the newly approved **2026 *American Mineralogist* inclusions Bernwoodite (`IMA 2025-072`) and Kopylovite (`IMA 2025-084`)**!
🔬 Berman–Simon Phase Boundary, Photoelastic Tatami Retardation & Ascent Equations
- Berman–Simon (1955) Diamond–Graphite Equilibrium Phase Boundary Pressure (Peq in GPa):
P_eq(GPa) = 1.55 + 0.0025 · T(°C)→ At1,200 °C, requiresP ≥ 4.55 GPa (>145 km Depth)! - Crossed-Polarizer Photoelastic Stress Retardation (Γnm) Producing "Tatami" Interference Colors:
Γ_nm = d_path · C_photoelastic · (σ_1 − σ_2)|I_XPL = I₀ · sin²(2θ) · sin²(π · Γ / λ) - Arrhenius Graphitization Rate (kgraph) During Slow Magma Ascent vs. Fast Kimberlite Quench:
k_graph(T) = A · exp( −E_act / (R · T) )(WhereE_act = 730 kJ/molfreezes completely below700 °C!)
Complete Reference Chart: 8 Natural vs. Lab Diamonds, Super-Deep Inclusions & 2026 IMA Species
| Diamond / Inclusion Type | Crossed-Polarizer (XPL) Pattern | Depth & Pressure | Diagnostic Gem / 2026 ID Test |
|---|---|---|---|
Natural Lithospheric & Type IIa DiamondC (150–250 km Craton Root) |
Crosshatched "Tatami" Glide-Plane Strain! |
150 – 250 km (5.0 – 8.0 GPa) |
Intersecting `{111}` slip bands + N3 (415nm) blue UV glow; repelled by N52 magnet |
HPHT Lab-Grown DiamondC + Fe-Ni-Co Metallic Flux |
ZERO Tatami Strain! (Uniform Dark Field) |
Lab Press (~5.5 GPa at 1,400°C for Days) |
Attracts to N52 Magnet (if flux present) + strong 254nm SWUV yellow-blue phosphorescence! |
CVD Lab-Grown DiamondC (Microwave Plasma Gas) |
Parallel Columnar / Brush Striations! |
Low-Pressure Gas (<0.1 atm CH4 + H2) |
Layer-by-layer growth striae + orange/red 737nm Silicon-Vacancy (`SiV⁻`) doublet |
660km Blue Ringwoodite Inclusionγ-(Mg,Fe)₂SiO₄ (1.5 wt% H₂O!) |
High-Stress Radial Birefringent Halo! |
525 – 660 km (18 – 24 GPa) |
Juína (Brazil) & Orapa (Botswana); proved Earth's Transition Zone holds 3× ocean water! |
Slow-Magma Graphitized Diamondsp³ Diamond → sp² Graphite |
Opaque Black Octahedral Shell |
Ascended Too Slowly (<4 GPa at 1,100°C) |
Beni Bousera (Morocco) & Ronda (Spain); proves why kimberlites must erupt at >50 km/h! |
Davemaoite InclusionCaSiO₃ (Cubic Pm3̄m Perovskite) |
Locked at >6 GPa Residual Internal Pressure |
660 – 900 km (24 – 35 GPa!) |
Orapa (Botswana · IMA 2020-012a); amorphizes into glass if freed from its diamond cage! |
Bernwoodite (Oct 2026 IMA)Ca₅TiAl₂Si₂O₁₄ (IMA 2025-072) |
Monoclinic `C2/c` Inclusion + Elastic Strain Halo |
>300 – 600 km Sublithospheric Mantle |
Published Oct 1, 2026 in American Mineralogist Vol. 111(10); Juína, Brazil; honors Bernard Wood |
Kopylovite (2026 Am. Min.)IMA 2025-084 (Sloan Kimberlite) |
Primary Kimberlite Diamond Inclusion |
Deep Cratonic Mantle Lithosphere |
Published Feb 2026 in American Mineralogist Vol. 111(2); Colorado-Wyoming State Line |
Frequently Asked Questions
How can I build a Crossed-Polarizer "Tatami" Strain Tester at home for under $10?
All you need is a **white LCD computer monitor or tablet screen** (which already emits 100% linearly polarized light!) and a single pair of **non-mirrored polarized sunglasses** (or a circular camera polarizing filter reversed). Place your diamond in a small glass dish of water or baby oil directly on the white screen (to eliminate facet surface reflections), look through the polarized sunglasses lens with a 10× loupe, and rotate the sunglasses until the background turns black (90° crossed). Inside a **Natural Diamond** (especially Type IIa or Ia), you will see weaving cross-hatched grey and rainbow **"Tatami" bands**! Inside an **HPHT Lab-Grown Diamond**, the interior stays uniformly dark with zero crosshatching.
Why do world-record giant diamonds (like the 3,106-carat Cullinan or Lesotho Promise) come from 660 km deep instead of normal 150 km depth?
In 2016–2026, GIA researchers led by Evan Smith proved that the world's largest, purest Type IIa diamonds (**"CLIPPIR" diamonds**—Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed) did not form in the normal 150–200 km continental root! Instead, they crystallized in **puddles of liquid metallic iron-nickel-carbon-sulfur alloy** at depths of **360 to 750 km** (in the Transition Zone and Lower Mantle), encapsulating tiny droplets of native iron, cohenite (`Fe3C`), and high-pressure silicates like **Davemaoite** and **2026 Bernwoodite**!
What happens if you break open a Super-Deep Diamond to take out a Davemaoite or Ringwoodite crystal?
Because Diamond is the stiffest, most incompressible material in nature (bulk modulus `K = 443 GPa`), a super-deep inclusion trapped inside a diamond at 660 km depth (`24 GPa`) is still held under **3 to 8 gigapascals (30,000 to 80,000 atmospheres) of residual confining pressure** even when sitting in a jeweler's showcase at sea level! If you polish into or crack open the diamond cage, the pressure drops to 1 atm and ultra-deep perovskite minerals like **Davemaoite (`CaSiO3`)** instantly expand and shatter into amorphous glass!