Diamond　is　an　ideal　material　for　a　broad　range　of　current　and　emerging　applications　in　tribology,　quantum　photonics,　high-power　electronics,　and　sensing.　However,　top-down　processing　is　very　challenging　due　to　its　extreme　chemical　and　physical　properties.　Gas-mediated　electron　beam-induced　etching　(EBIE)　has　recently　emerged　as　a　minimally　invasive,　facile　means　to　dry　etch　and　pattern　diamond　at　the　nanoscale　using　oxidizing　precursor　gases　such　as　O-2　and　H2O.　Here　we　explain　the　roles　of　oxygen　and　hydrogen　in　the　etch　process　and　show　that　oxygen　gives　rise　to　rapid,　isotropic　etching,　while　the　addition　of　hydrogen　gives　rise　to　anisotropic　etching　and　the　formation　of　topographic　surface　patterns.　We　identify　the　etch　reaction　pathways　and　show　that　the　anisotropy　is　caused　by　preferential　passivation　of　specific　crystal　planes.　The　anisotropy　can　be　controlled　by　the　partial　pressure　of　hydrogen　and　by　using　a　remote　RF　plasma　source　to　radicalize　the　precursor　gas.　It　can　be　used　to　manipulate　the　geometries　of　topographic　surface　patterns　as　well　as　nano-　and　microstructures　fabricated　by　EBIE.　Our　findings　constitute　a　comprehensive　explanation　of　the　anisotropic　etch　process　and　advance　present　understanding　of　electron-surface　interactions.