When you hold this mystery of the Zone in your hands, you feel how your skin becomes tougher and less sensitive. You can still shoot through it with a bullet, but it's much more difficult to cut through - In-game description
The Kolobok increases rupture protection. In itself this advantage is very minor, however considering it has no downside whatsoever, it is an effective artifact to use.
A relatively rare and expensive artifact, which forms in chemically active areas. Highly valued for its ability to increase the body's overall well-being. Emits radiation. - In-game description
The Kolobok returns in S.T.A.L.K.E.R.: Clear Sky, this time as an uncommon artifact. It provides a +4 in health regenration, and a +2 in radiation. The Kolobok can be helpful to allow the player to recover health over time, although an anti-radiation artifact will be needed.
A Kolobok can be found in the underground of the Mine Shaft in the Red Forest, in a chemical area in the Poltergeists zone - beware the Poltergeists are extremely dangerous and can easily kill the player. It can be found as well in the Agroprom Underground.
A relatively rare artifact which forms in areas contaminated with chemicals, Kolobok is highly valued for its ability to heal wounds of any kind in mere minutes. For unknown reasons, scientists are spreading rumors among stalkers that the artifact interacts with the bearer's genetic code. Emits radiation. - In-game description
The Kolobok may be found in chemical anomaly fields; the top of Oakpine Anomaly in Zaton and the Vine in Pripyat spawns one by default, in the latter either on the top floor of the apartment building or in the crater, often swapping places with a Firefly.
The Kolobok is sold by Beard after completing the Compass mission.
In Clear Sky and Call of Pripyat, the health regeneration equals out to 8 HP regenerated over a 2 'in-game minute' period (12 seconds) – in short, it takes a full in-game half an hour (3 realtime minutes) to regenerate a full health bar.