The cooling of solids by optical means only using anti-Stokes emission has a long history of research and achievements. Such cooling methods have many advantages ranging from no-moving parts or fluids through to operation in vacuum and may have applications to cryosurgery. However achieving large optical cryocooling powers has been difficult to achieve except in certain rare-earth crystals. Through study of the emission and absorption cross sections we find that diamond, containing either NV or SiV (Nitrogen or Silicon vacancy), defects shows potential for optical cryocooling and in particular, NV doping shows promise for optical refrigeration. We study the optical cooling of doped diamond microcrystals ranging 10−250 µm in diameter trapped either in vacuum or in water. For the vacuum case we find NV-doped microdiamond optical cooling below room temperature could exceed |∆T | > 10 K, for irradiation powers of Pin < 100 mW. We predict that such temperature changes should be easily observed via large alterations in the diffusion constant for optically cryocooled microdiamonds trapped in water in an optical tweezer or via spectroscopic signatures such as the ZPL width or Raman line.Optical refrigeration, or optical cryocooling, uses antiStokes emission in solids to deplete the phonon population within the solid, cooling it. First suggested by Pringsheim 1 , and initially demonstrated by Epstein et al in 19952 , the phenomena is observed when low entropy laser light is primarily absorbed at wavelengths slightly longer than the mean fluorescence wavelength (λ f ) of the material. The light is reemitted with a broadband fluorescence possessing a mean energy which is higher than the incident pump laser. The increase in energy is due to absorption of lattice phonons (vibrations), reducing the net temperature of the solid. Optical refrigeration has experimentally achieved temperatures of T ∼155K (from room temperature) using ytterbium-doped fluoride crystal (YLiF 4 : Yb 3+ ) 3 , T ∼250K (from 290K), for the semiconductor CdS 4 , and T ∼114K more recently using a multi-pass setup with Yb-doped crystals 5 . These temperatures outperform Peltier/thermoelectric coolers and reach into the defined cryogenic regime (<123K). Importantly they operate with no mechanical vibrations, magnetic/electric fields or moving mechanical/liquid/gas components and are ideal refrigeration solutions in many difficult/sensitive situations eg. optomechanical, space, sensing experiments 6-9 . In addition, spin properties of diamond defects have recently attracted much attention owing to their long spin coherence times T 2 . Since T 2 times increase at low temperatures, a new way of cooling diamond is important for applications where long spin relaxation times are needed. Microscopic diamond crystals are highly biocompatible and can be functionalised to attach to specific biologically important ligands 10 . Cryosurgery and cryotherapy is a method to destroy diseased or harmful tissues within the body and involves cyclic freezing and thawing of the ...