Introduced here is a newly discovered, hard, high-carbon, macroscopically homogenous, solid lignomor that is part of the natural decomposition product of mature, rainforest hardwood trees of the species myrtle beech (Nothofagus cunninghamii (Hook. f.) Oerst.). The substance, named cunnite, exhibits conchoidal fracture (indicating isotropic bonding), whereas all other tree decomposition products noted were readily friable, with no evidence of strong bonding. It is a dehydrated form of lignomor and its pre-dehydrated form also has conchoidal fracture. Cunnite was examined by carbon assay, radiocarbon dating, scanning electron microscopy (SEM) and Fourier transform infra-red spectroscopy (FT-IR). All wood cell walls had decomposed but the FT-IR results suggested that cunnite is high in lignin, and therefore it is most likely formed by a brown-rot fungi. Cunnite appears to be unique in that it is a product of natural decomposition but has relatively strong bonding. This bonding may be either from re-polymerisation or hydrogen bonding. If original polymer segments remain then they are most likely often cross-linked. Although cunnite appears to be stable beyond the decomposition of the entire tree, the age determined from radiocarbon dating for our samples does not imply that it is particularly long-lived thereafter. Cunnite is part of the forest carbon cycle but forms only a small fraction of the soil organic carbon pool. If the composition of cunnite is determined accurately in future work, and a synthetic version made, then it may be useful on a commercial scale as an adsorbent of hydrogen (i.e. storage for renewable energy) or of toxic substances; or it may provide other products useful in materials science or chemical engineering.