Lonsdaleite: When Hexagonal Carbon Rivals Diamond

A Long-Debated Carbon Allotrope

Since 1967, a mineral has intrigued the scientific community: lonsdaleite, also known as hexagonal diamond. First identified in fragments of the Canyon Diablo meteorite in Arizona, this form of pure carbon differs from conventional diamond through its hexagonal crystal lattice (wurtzite type), whereas classic diamond adopts a cubic structure.

It is named after Kathleen Lonsdale (1903–1971), an Irish crystallographer whose pioneering work in X-ray diffraction transformed modern crystallography.

Natural Formation: Born from Meteorite Impacts

In nature, lonsdaleite forms when graphite within meteorites undergoes pressures exceeding 12 GPa and temperatures approaching 2,000 K during planetary impact. This transformation converts graphite’s layered structure into a hexagonal carbon lattice within seconds. Natural specimens remain microscopic, however, and typically contain structural defects that give them an effective hardness estimated at only 7 to 8 on the Mohs scale, well below that of diamond.

A Fifty-Year Scientific Debate

The very existence of lonsdaleite as a distinct phase has been the subject of major controversy. In 2014, Németh and colleagues published a study in Nature Communications demonstrating, through ultra-high-resolution electron microscopy, that samples identified as lonsdaleite were in fact cubic diamond with stacking faults and twins. This analysis led some researchers to propose the term “stacking-disordered diamond” as a more accurate description of the material.

2025: First Bulk Synthesis

In 2025, an international team led by Liuxiang Yang at the Center for High Pressure Science and Technology Advanced Research (HPSTAR) in Beijing published in Nature (vol. 644, pp. 370–375) the first successful bulk synthesis of hexagonal diamond. By compressing single-crystal graphite under high-pressure, high-temperature, quasi-hydrostatic conditions, the researchers produced millimeter-sized lonsdaleite crystals of high purity. This achievement provided a concrete answer to the debate by demonstrating that hexagonal diamond can exist as a distinct phase and be synthesized reproducibly.

Hardness: Theoretical Predictions vs. Measured Values

Computational simulations have predicted that ideal lonsdaleite could reach a hardness up to 58% greater than that of cubic diamond. This value remains theoretical, however, and is based on models of perfect crystal structures.

Actual measurements on the 2025 synthesized samples show a Vickers hardness of 114 ± 6.4 GPa along the axial direction. For comparison, natural diamond measures approximately 110 GPa on the (110) face under equivalent testing conditions. The synthesized lonsdaleite also exhibits a Young’s modulus of 1,229 ± 15 GPa, confirming greater stiffness than cubic diamond. The measured hardness difference is real but modest, far from the theoretical predictions.

Properties and Potential Applications

Lonsdaleite has a refractive index of 2.40 to 2.41 and a specific gravity of 3.2 to 3.3. Its thermal stability is notable: oxidation begins only at approximately 848 °C, later than for other forms of diamond. These properties, combined with its high stiffness, open possibilities for ultra-resistant cutting tools, protective coatings for extreme environments, and next-generation abrasives. Scaling commercial production remains an active challenge.

Gemological Relevance

Lonsdaleite does not currently appear in the gem trade: natural specimens are microscopic and synthetic production remains limited. Its existence nonetheless reminds us that our understanding of carbon allotropes continues to evolve. The same element that forms graphite, diamond, and fullerenes can also produce a hexagonal structure with distinct mechanical properties.

At Laboratoire Gem Quebec, we follow these advances closely because they enrich the scientific foundations upon which gemological expertise is built.