Diamond is not just a perfect gemstone. The tiny imperfections inside diamond can be turned into ultrasensitive nanoscale quantum sensors which can offer brand-new lenses to see through intricate phenomena spanning from atomic and molecular objects to events on a grand scale. In this talk, we will start with an overview of quantum sensing technologies based upon spin defects (e.g. nitrogen-vacancy centers) in diamond. We will then discuss our recent efforts at WashU to employ these diamond sensors for a wide range of applications covering condensed matter physics, biomedical imaging and geoscience. If time permits, we will present some of our results on developing a new generation of quantum sensors beyond diamond, specifically in two-dimensional materials.