Progress in nanodiamond technologies allows regulate diamond surface properties, amounts of grafted functional groups, and positive (negative) potential of particles in aqueous media. It stimulates the applications of diamonds as advanced nanoplatforms with chemical and radiation resistivity (laser light, UV, X- and γ-rays) and luminescent properties useful for catalysis and biomedicine. The modification of diamonds with fullerenes C60, C70, and endofullerenes M@C2n (n ≥ 30) with captured magnetic metal atoms (M) of 4f and 3d elements is a prospective way to create medical preparations based on carbon and metal–carbon structures as active scavengers of free radicals, photosensitizers, minimally toxic, and effective contrasting agents for MRI diagnostics owing to magnetic atoms encapsulated inside firm carbon cages. Authors have developed the synthesis of fullerenes and endofullerenes with 4f, 3d elements and found new ways to transform pristine carbon structures to water-soluble fullerenols by two-stage hydroxylation. The studies of fullerenols and endofullerenols by optical absorption, Raman spectroscopy, small-angle neutron and synchrotron radiation scattering, and other methods have confirmed their expected structure, the coordination of atoms, and showed fractal molecular ordering in aqueous media. Further, taking various proportions of components, the authors prepared the complexes by the association of electronegative Gadolinium fullerenols with diamonds carrying positive charges. The stability of such structures was proved during cyclic temperature variation (25–70–25 °C) when their ordering in solutions was detected at nanoscales by X-ray scattering. Following NMR measurements (25 °C) on protons in these aqueous systems allowed to find longitudinal (T1) and transversal (T2) relaxation times: T1 < T2 in pure diamond dispersion, T1 ≤T2 in fullerenol solution, but T1 «T2 in the dispersion of complexes. Thus, by complexing there were prepared so-called negative contrast agents very needed in MRI practice. Final biological tests on cell cultures showed low toxicity of complexes that is desirable for the implementation in theranostics.