The least compressible material known, with a bulk modulus of 491 gigapascals (GPa) is aggregated diamond nanorods. Aggregated diamond nanorods are what you get when you compress buckytubes, an allotrope of carbon, at 20 GPa, under a temperature of 2500 Kelvin, using a 5000 metric ton anvil press designed specifically for the purpose. The end result is a spaghetti of interconnected diamond nanotubes, with diameters between 5 and 20 nanometers and lengths of about a micrometer each.

A bulk modulus of aggregated diamond nanorods, 491 GPa, is about three times of that of iron, which is 160 GPa. Conventional diamond has a bulk modulus of 442 GPa. A bulk modulus of 491 GPa means that a pressure increase of 49.1 GPa would be necessary to compress the aggregated diamond nanorods by a factor of 10%. A 98.2 GPa pressure increase would be necessary to increase it by 20%, and so on. For comparison, pressure at sea level (one atmosphere) is equal to about 100 kPa (kilopascals), and every ten meters of descent in the ocean increases pressure by another atmosphere. So the lowest point in the ocean, about 10 km before the surface, experiences a pressure increase of 100 MPa over the surface, which would be enough to compress a solid sphere of aggregated diamond nanorods by a factor of .002%.

Aggregated diamond nanorods get their strength from the famous sp² bond, the strongest bond in all of chemistry. The sp² bond is a carbon-carbon bond, also found in graphite (along atom-thick 2D planes), in diamond (3D), and carbon nanotube fiber (which has been proposed as a building material for space elevators). The sp² bond is not just the strongest known chemical bond, it is the strongest possible chemical bond, so aggregated diamond nanorods may not only be the least compressible material we currently know, but the least compressible material, period. The only exception may be the exotic matter inside of super-dense stars, sometimes called neutronium.

Remarkably, aggregated diamond nanorods are even denser than pure diamond by a factor of 0.3%. Its appearance is dark with a slight rainbow-like scattering of light, similar to an oil slick on asphalt. Other substances less compressible than diamond, but more so than aggregated diamond nanorods, include type IIa diamond and ultrahard fullerite.