Abstract: I will describe recent work on the development of a sensitive magnetic-field microscope with sub-micrometer spatial resolution. This instrument employs a layer of nitrogen-vacancy (NV) color centers at the surface of a diamond chip to enable imaging of magnetic fields from nearby samples. Together with collaborators in the MIT department of Earth, Atmospheric and Planetary Sciences, we have used the NV-diamond microscope to study distributions of magnetic minerals in a variety of terrestrial and meteoric rocks. The combination of spatial resolution and magnetic sensitivity of our instrument permits magnetic analyses of previously inaccessible geologic samples in which the regions of interest are mixed with undesirable magnetic field sources at the <<100 µm scale. We employed this technique to map magnetic fields in chondrules from the Semarkona meteorite, helping to constrain the magnitudes of nebular magnetic fields which likely played a key role in accretion disk dynamics during the formation of the solar system.