The Stern-Gerlach effect, discovered a century ago, has become a paradigm of quantum mechanics. Surprisingly, until recently, there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Indeed, several theoretical studies have explained why a Stern-Gerlach interferometer is a formidable challenge. Here we provide a detailed account of the realization of a full-loop Stern-Gerlach interferometer for single atoms, and use the acquired understanding to show how this novel setup may be used to realize an interferometer for macroscopic objects doped with a single spin. Such a realization would open the door to a new era of fundamental probes, including the realization of new tests at the interface of quantum mechanics and gravity.