This is the Chaos Star pendant.
Chaos, also known to its servants as the Primordial Truth or the Primordial Annihilator, and to the Imperium of Man as the Archenemy, is the universal and usually malign spiritual force embodied by the malevolent intelligent entities comprised of psychic energy that live in the Warp. Chaos is the equal and opposite of Order. The entities who embody Chaos are mostly daemons, but the term also encompasses those mortals who have thrown in their lot with Chaos, ranging from simple peasants and/or manufactorum labourers who serve as Chaos Cultists, to Traitor Imperial Guardsmen, planetary militia, Imperial nobles, Planetary Governors, and even the mighty Chaos Space Marines and Traitor Titan Legions of the Dark Mechanicus.
This item is made from brass. Brass is very resistant material.
You can wear the pendant while taking a shower or swimming.
Comes with a bronze colored chain that is 22 inches long.
I's diameter is 1.1 inches / 28mm.
The black look is given by patination.