Carbonado is diamond, but not a diamond in the jeweler’s sense. It is polycrystalline: a tightly bonded mass of randomly oriented microcrystals, typically cubes five to twenty micrometers across set in a matrix of still finer diamond[1,2]. X-ray diffraction of crushed grains looks like ordinary diamond, and its hardness is similar, perhaps slightly greater[1]. Because its crystallites interlock in every direction, carbonado is tougher than single-crystal diamond, so tough that it can be cut only by laser[1].
Color ranges from black and putty gray to brown, deep purple, pink, rusty red and occasional olive green[1]. The material is opaque; Haggerty attributes this to the random orientation of the crystallites, which scatter and absorb light[1]. Trueb and De Wys, who described carbonado as natural polycrystalline diamond in 1969, compared its texture to synthetic sintered diamond[2,1].
Its most distinctive feature is porosity. Pores occupy an estimated five to fifteen percent of the volume, are spherical or oblate, and persist into the interior[1]. Densities run from about 2.8 to 3.45 grams per cubic centimeter, most near 3.05, against 3.52 for gem diamond[1]. Haggerty likens the pores to the gas vesicles of basalt that degassed near the surface, and argues that such voids are incompatible with the high pressures under which mantle diamonds grow[1]. Surface pores are often filled with low-temperature minerals such as quartz, goethite and goyazite, which record a later, oxidizing history at Earth’s surface[1].
Many stones also wear a glassy patina, a veneer of diamond only micrometers thick, dimpled, furrowed and marked by mounds and flow structures that resemble volcanic melts or metallurgical slag[1]. Whether this skin formed directly from the porous body beneath or was deposited later remains open; frictional melting has also been proposed[1].
Inside, carbonado lacks the inclusion suite of conventional diamonds. In their place are metals (iron, nickel, chromium, titanium), alloys, moissanite and osbornite[1]. A 2016 congress abstract argued that osbornite implies an extraterrestrial origin[3]; a separate study of titanium–copper nitrides in carbonado favored a hot, deep-mantle setting instead[4]. Nanometer-scale inclusions, including garnet, have been reported and read by some researchers as crustal[5]. Both nitride reports are conference abstracts, not peer-reviewed papers, and are cited here with that caution.
Comparison with the other polycrystalline diamond, framesite, sharpens the contrast. Framesite is known to originate in the mantle and appears to have crystallized rapidly shortly before the kimberlite eruptions that carried it upward[6]. Its open spaces lie between adjoining crystal faces and are irregularly polyhedral, whereas carbonado’s pores are rounded[1]. Other polycrystalline varieties have micrometer-wide radial or parallel openings rather than spherical voids[1].
The patina also affects simple instruments. Haggerty reports that standard thermal-conductivity diamond testers respond sharply to glassy diamond surfaces, less so to ridges and mounds, and only sluggishly and erratically to pore-rich surfaces, possibly because of discontinuities at the boundaries between microcrystals[1]. A tester reading on any stone is therefore no substitute for laboratory analysis.
Taken together, the anatomy is unusual on every count, but it does not by itself name a birthplace. In the space-origin view, vapor-deposited diamond cemented the microcrystals, escaping hydrogen opened the pores, and the patina is a high-vacuum melt product[1]. In terrestrial models, fluids or radiation do comparable work[7,8]. The material is better documented than its origin.