§1 — Why diamond as an anvil?
Diamond has a unique combination of properties that makes it the only viable material for an anvil able to exceed 100 GPa: maximum hardness (Vickers ~10,000 HV), an exceptional bulk modulus (K₀ ≈ 446 GPa [Occelli et al. 2003]), and an unmatched electromagnetic transmission window — from the UV (~225 nm) to the far-IR (~300 μm), with only a two-phonon absorption band between 1800 and 2500 cm⁻¹. In practice, two opposing anvils form a sample chamber a few tens of microns across, compressed by the culets. The maximum attainable pressure is set by the culet diameter, the bevel geometry and the diamond quality [O’Bannon et al. 2018, Rev. Sci. Instrum. 89:111501].

§2 — Cut geometries
Four culet profiles cover most experimental needs. See the SVG schematics below.
• Standard / Modified Brilliant Cut (MBC): 16-facet modified brilliant cut, 3.75 mm stone diameter, flat culet 100–600 μm, (100) orientation. General use <100 GPa. Typical supplier: Almax easyLab [almax-easylab.com].
• Boehler-Almax (BA): 70° or 80° conical profile, 2.50–3.10 mm diameter, enlarged X-ray aperture (up to 80° half-angle). De facto standard for synchrotron diffraction [almax-easylab.com/BA; Boehler & De Hantsetters 2004].
• Single & double bevel: inner culet 20–100 μm, 8–10° bevel out to 300–350 μm. Routinely exceeds 200 GPa, up to ~400 GPa with a double bevel [O’Bannon 2018].
• Toroidal (t-DAC): toroidal groove FIB-milled around a 9–30 μm central culet. The groove traps gasket material and supports the anvil, reproducibly reaching 4–6 Mbar [Jenei et al. 2018, Nat. Commun.; Dewaele et al. 2018, Nat. Commun.].

§3 — Diamond types: classification and properties
Diamond anvils are classified by their impurity content [Diamond type classification; Almax easyLab FAQ]:
• Type Ia: ~98% of natural production. Aggregated nitrogen (IaA, IaB >100 ppm). Strong IR absorption between 1000 and 1450 cm⁻¹ (N impurities) and in the far-IR. Moderate visible fluorescence. Use: Raman, XRD, high pressure. Above 2700 cm⁻¹ the transmission is better than type IIa.
• Type Ib: ~0.1% of natural diamond, dominant for industrial HPHT synthetics. Dispersed atomic nitrogen (~100 ppm), yellow colour. Marked IR absorption at 1344 cm⁻¹ (synthetic indicator).
• Type IIa: ~2% of natural diamond, <1 ppm N. Transparent from 225 nm to ~300 μm except the two-phonon band. Very low fluorescence. Reference for IR and Raman in the DAC. Ultra-low birefringence available [Crystran].
• Type IIas (synthetic HPHT): synthesised at 5–6 GPa / 1300–1600°C. <1 ppm N, reproducible IIa optical quality, excellent uniformity. Available in large sizes (up to 5×5 mm). Ultra-low Raman fluorescence [Almax FAQ].
• Type IIac (synthetic CVD): chemical-vapour deposited. Excellent for IR (lower far-IR absorption than natural IIa). High Raman fluorescence (avoid for Raman). Recommended for IR spectroscopy and XRD at high pressure [Almax FAQ].

§4 — Nano-polycrystalline diamond (NPD) — Ehime University / GRC
NPD is synthesised by direct conversion of graphite at high pressure and temperature (15 GPa, 2300–2500°C) in the 6000-tonne BOTCHAN-6000 multi-anvil press at the Geodynamics Research Center (GRC), Ehime University, Japan [Ishimatsu et al. 2012, J. Synchrotron Radiat. 19:768; Nakamoto et al. 2011, Rev. Sci. Instrum. 82:066104].
Distinctive properties:
• Randomly oriented grains a few tens of nanometres in size → no cleavage plane, no hardness anisotropy
• Higher hardness than single crystal for culets >300 μm (attainable pressure 1.5 to 2× higher)
• Optically transparent (comparable to type Ib), usable for IR and spectroscopy
• No Bragg glitches in X-ray absorption spectroscopy (XAS/XANES): the signal is no longer perturbed by single-crystal diffraction peaks
NPD is available on request from the GRC for partnership experiments, and sold in Japan by Sumitomo Electric (SUMICRYSTAL/SUMIDIA).

§5 — Culet and maximum pressure: the empirical law
O’Bannon et al. (2018) compiled the full literature dataset (flat, bevel, double bevel) and fitted a P_max(d_culet) relation [doi:10.1063/1.5049720]. Key results:
See the interactive chart below. Above ~300 GPa, conventional bevel geometries must give way to toroidal anvils (t-DAC) or double-stage systems (ds-DAC) using NPD or NCD as the second stage.
Typical parameters for ~400 GPa: 20 μm culet, 8.5° bevel, bevel/culet diameter ratio = 14–18.

§6 — Selection by experimental technique
The diamond type to choose depends on the technique:
• Raman: Type Ia (low fluorescence) or IIas (ultra-low). Avoid IIac (CVD fluorescence too high at 532 nm).
• IR (FTIR) >2700 cm⁻¹: Type Ia acceptable (better transmission than IIa above 2700 cm⁻¹ [Haas & Daniels 1983, Appl. Spectrosc. 37:284]).
• IR (<1200 cm⁻¹, far-IR): Type IIa or IIac essential (Type Ia absorbs strongly <1200 cm⁻¹ through nitrogen).
• Synchrotron XRD: Type Ia (Boehler-Almax) or IIas. NPD is ideal for XANES (no glitches).
• UV (<225 nm): Type IIa only (Ia absorbs below 250 nm). Type IIa transmits down to ~225 nm.
• Very high pressure (>200 GPa): IIas HPHT or NPD for increased strength.