Nanodiamonds (ND), i.e., sp(3)-hybridized nanoscale carbon particles, are being widely explored for biomedical applications, such as drug delivery and medical imaging, because of their biocompatibility, nontoxic nature, and ease of surface functionalization. However, little is known about the colloidal and rheological properties of ND dispersions. Here, we report a systematic study on NDs dispersed in a nonpolar liquid, mineral oil. We find that our smallest entities in dispersion are tightly bound aggregates (similar to 400 nm) of individual primary (similar to 5 nm) particles. These aggregates form colloidal gels (with frequency-independent moduli) at low particle concentrations (similar to 5wt. %). Gelation is likely due to attractive interparticle forces composed of both van der Waals and hydrogen-bonding interactions. The elastic modulus (G'), yield stress (sigma(y)), and yield strain (gamma(0)) of these colloidal gels all show scaling relationships with ND concentration, with G' and sigma(y) exhibiting positive power-law exponents and gamma(0) showing a negative one. These results suggest a sample-spanning network of interconnected flocs, each of which is composed of several ND aggregates. Functionalization of ND surfaces with methacrylate groups eliminates gelation and gives a stable, low-viscosity dispersion. Much like other particulate gels, ND gels show thixotropic behavior, i.e., the gel network is disrupted by large deformations (steady or oscillatory shear) and is reformed upon cessation of shear. However, after oscillatory shear at a large strain-amplitude (1000%), the recovery is incomplete and the modulus of the recovered gel is only half its original value. In contrast, near-complete recovery of the modulus is observed after steady shear. (C) 2014 The Society of Rheology.