The Shell is an artifact that can be found only in S.T.A.L.K.E.R.: Clear Sky and S.T.A.L.K.E.R.: Call of Pripyat.
"This artifact used to be considered trash, devoid of any useful properties. However, scientists recently discovered that if kept in constant contact with the body, it has an excellent stimulating effect on the nervous system, replenishing the bearer's energy. Emits radiation." - Clear Sky description
"This artifact used to be considered trash, devoid of any useful properties. However, scientists recently discovered that if kept in constant contact with the body, it has an excellent stimulating effect on the nervous system. Emits radiation." - Call of Pripyat description
This artifact's appearance is clearly a nod and homage to the 'empties' as described in the novella which inspired the S.T.A.L.K.E.R series, 'Roadside Picnic'. They're described as follows: "They're just two copper disks the size of a saucer, about a quarter inch thick, with a space of a foot and a half between them. There's nothing else. I mean absolutely nothing, just empty space. You can stick your hand in them, or even your head, if you're so knocked out by the whole thing – just emptiness and more emptiness, thin air. And for all that, of course ,there is some force between them, as I understand it, because you can't press them together, and no one's been able to pull them apart, either."
Printed mine with a semi-transparent PLA middle. Unfortunately the clear filament suggested, Watson, is too expensive to ship from Russia. Still, excellent model.