In addition to being the traditional token of marital intent, the diamond has long provided the—ahem—gold standard for super-hard materials. But physicists at Shanghai Jiao Tong University and at the University of Nevada, Las Vegas, say that two lesser-known materials, wurtzite boron nitride and lonsdaleite, are even harder.

Wurtzite boron nitride, the researchers calculate, resists indentation with 18 percent more fortitude than its glittering counterpart, whereas lonsdaleite, a hexagonal lattice of carbon atoms also known as hexagonal diamond, boasts indentation strength a whopping 58 percent greater than that of diamond. That would be a new record for hardness, the team claims in the study, published this month in the journal Physical Review Letters.

There's a catch to the claims, however: both materials are quite rare, and the assessment of their hardness was determined by calculations rather than physical demonstrations. Natalia Dubrovinskaia, a crystallographer at the University of Bayreuth in Germany, told the New Scientist that although the new work is important, it remains unproven until the materials can be physically isolated and tested.

In a 2004 open letter to the materials science community published in Nature Materials, Dubrovinskaia and an international team of colleagues cautioned against making claims of "harder than diamond" materials. The letter's authors specifically cite the pitfalls of basing such proclamations on simulated models of material structure, which might ignore real-life impurities or other subtleties: "The relationship between the computed microscopic behavior of these materials and experimental measurements of their bulk properties, such as hardness, strength, toughness and abrasion resistance, is less than clear."

Diamond photo by Mario Sarto via Wikimedia Commons