The Mantle of Stars is an armor added by Astral Sorcery. In addition to serving as an armor piece slightly worse than a Diamond Chestplate, but with far better enchantability, it can be attuned to one of the constellations to provide additional effects.
This constellation will completely block an attack inflicted upon the player. By default the Mantle can block 3 attacks and recharge one stack every 80 ticks (or 4 seconds). This can be changed in the mod's config file.
When worn, the Mantle allows the wearer to glide as if using an Elytra.
Astral Tome entry
Mundane armor like leather and iron certainly serve their purpose, however, they are neither particularly beautiful nor very functional. Infusing a normal leather chest with the energetic properties of the Resonating Gem, and the transmissive aspect of Stardust, allow for a more elegant and suitable piece of armor, rather than the crude attire of a brute.