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Cosmic Ring - adjustable in size
 
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Ring made out of brass (nickel-free) 
Gemstone: Moonstone
Available in brass gold and silver
 
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The Ring itself is pretty resistant. You can even wear it under water and in the ocean, but be aware that the shiny brass will change it's colour faster when doing so. After time the brass with get a little dull, more like a vintage look - but still beautiful to wear. 

Also be careful with the gemstone. The gemstone is glued to the brass, so you can loose it more easily when going into the water. 
 
Your fingers can turn green (especially if you are in very hot weather and your fingers are sweaty). But this only happens in very rare cases.