A Rosarius is a symbol of office for the clerical members of the Adeptus Ministorum and is also a symbol of office for the Chaplains of the Space Marines alongside the Crozius. A Rosarius normally takes the form of a gorget, amulet, or signet ring of adamantium usually shaped into the form of the Imperial Aquila or a Gothic Cross with a jewel in the centre, and houses a powerful protective Conversion Field emitter, protecting the Ministorum's priest or the Space Marine Chaplain from attacks both physical and psychic that might otherwise break through the protection of even the mighty Space Marine Power Armour or the indomitable psychic strength of his own will. The field's effect is to convert the kinetic energy of an impact into harmless photons of the visible light spectrum. When the field stops a shot, a blinding flash of light is produced as the energetic conversion occurs. The field is capable of rendering even Plasma Gun shots harmless. It is worn around the neck or waist on a sash, a cord, or a string of prayer beads. A Rosarius does not give off the same glow as a Refraction Field. However, they do convert some of the energy they observe into visible light, causing them to flash and spark when they are hit. Space Marine Chaplains are always granted a Rosarius upon attaining that rank within their Chapter.
The most noted bearers of Rosarii are Space Marine Chaplains who receive them from the Ecclesiarchy as a symbol of the link they share in devotion to the Emperor of Mankind. Ecclesiarchy Confessors and Missionaries also wear a Rosarius. Inquisitors sometimes carry Rosarii as well, although this is probably out of practical concerns to show their faith in the Emperor and to other Imperial Cult believers rather than as a show of allegiance to the Ecclesiarchy. Also, a large number of the Sisters of Battle carry a Rosarius due to the strong affiliation between the Adepta Sororitas' Orders Militant and the Adeptus Ministorum. The Wolf Priests of the Space Wolves Chapter, who combine the roles of Chaplain and Apothecary in that non-standard Chapter's organisation, are equipped with Wolf Amulets, which confer many of the same benefits as the Rosarius. The Rosarius is often referred to in the Ecclesiarchy's sacred texts as the Soul's Armour. Many of the Ecclesiarchy's Missionaries and the Space Marines' Chaplains have been saved from death or corruption by the Rosarius' Conversion Field.
- Codex: Space Marines (5th Edition), pg. 58
- Deathwatch: Rites of Battle (RPG), pg. 143
- Codex: Space Wolves (5th Edition), pg. 35