The following text was taken from the 'Roadside Picnic' Wikipedia page before it was removed on the grounds that it was "superfluous trivia". (13 May 2012)
Artifacts left by Visitors in the Zones
The artifacts left behind by the Visitors can be broken down into four categories:
(1) Objects beneficial to humans, yet whose original purpose, how precisely they work or how to manufacture them is not understood. The 'So-So' and 'Bracelets' are among the artifacts that fall into this category.
(2) Objects whose functionality, original purpose or how to use them to benefit humans can not yet be understood. The 'Black Sprays' and 'Needles' are among the artifacts that fall into this category.
(3) Objects that are unique. Their existence is passed along as legends by Stalkers; were never seen by scientists, whose functionality is so dangerous and so far beyond human comprehension that they are better off left undisturbed. The 'Golden Sphere' and the 'Jolly Ghost' are among the artifacts that fall into this category.
(4) Not object but effects on people who were present inside the Zones during the Visitation. Humans who survived the Visitation without going blind (apparently from a loud noise) or infected by the plague caused unexplained problems if they emigrated away. A barber who survived the Visitation emigrated to a far off city and within a year 90% of his customers died in mysterious circumstances as well as a number of natural disasters foreign to the area (typhoons, tornadoes) hit his city. Even people who were never present during the Visitation but frequently visit the Zone are changed somehow, for example by having mutated children or by having duplicates of their dead relatives return to their homes.
An entire economy has developed around the Zone in the trade of items obtained from the Zone by Stalkers. The only legal buyer of artifacts is the Institute for Extraterrestrial Cultures, but a large black market has also evolved. Stalkers will seek to gain the best price of artifacts while avoiding the police who enforce strict laws against the black market. The functioning of this market and the relative prices Stalkers are paid for different artifacts plays a key role in the plot. The novel also provides some idea for the relative markup of prices as items move from Stalkers to external markets like Europe, where the highest prices can be obtained, usually about five times what the Stalker was paid. Prices are often given in the novel but never in currency, just numbers. Other times prices are given in relative sums, for example a Stalker is paid two months salary as a bonus for getting a certain 'full empty' early in the novel. So the price of a battery paid to a Stalker is given as 20.
- Batteries - A round black stick (also called So-So) that produced endless energy and can be used to power vehicles instead of an engine. Small, easily portable, and able to replicate through a process similar to cell division. Its power to propel vehicles appears to last indefinitely. A Stalker is said to get 20 for on the market for a battery while the price in Europe is 100.
- Death Lamp - "Eight years ago a Stalker by the name of Stefan Norman, nicknamed Four-Eyes, brought out an apparatus from the Zone that, as far as can be judged, was some kind of ray-emitting system fatal to earth organisms. This Four-Eyes offered the apparatus to the Institute. They did not agree on the price. Four-eyes re-entered the Zone and never came out again. The present whereabouts of the apparatus is unknown. People at the Institute are still tearing their hair out over failing to buy it, and now are offering any amount for it that could be written on a check."
- Empties - Two copper discs the size of Frisbees, about a quarter inch thick, which permanently maintain an empty space of a foot and a half between each other. It is unknown how the two discs are attracted to each other or what holds them in place. No force seems to be able to push them closer together or pull them apart. It is possible to pass any object through the empty space between the two discs. The device seems to possess no other unusual properties and has the mass of two copper discs of the appropriate size. Possibly a type of a container. One unique specimen, the "full empty," has been found, "filled" with blue liquid. The blue substance sifted cloudily in slow streams between the discs, like a glass jar with blue syrup inside. The "full" specimen was too heavy for one strong man to move, and almost too heavy for two men to move. The novel states that a Stalker can get 400 for an empty while the price of an empty in Europe is said to be 2,500. A Stalker was legally able to get 2 months pay for a full empty.
- Itchers - A few centimetres in diameter. Squeezing it several times causes strange effects in a radius of a few hundred yards. Dogs start howling and barking as they sense its activation, before humans notice any effects. It affects humans in different ways: some get nose-bleeds, others starts hysterically screaming, some fall into deep depression, others go berserk, and some panic.
- Black Sprays - Black beads that are sometimes used in jewelery. A ray of light shone into one of these beads will be delayed in time and distorted. The transmission of the light is delayed depending on the bead’s weight, size, and several other parameters. The light which exits the bead is always less than what entered. No other known properties.
- Sponges - Mentioned, but never described in the novel.
- Pins - Slightly blue and occasionally spattered with other colors-yellow, red, and green. When squeezed with fingers, a few pins generated "weak red bolts illuminating the pin that were suddenly replaced by slower green pulses." The majority required special machines to cause this effect. Unknown functionality.
- Bracelets - Somehow causes the person wearing one to become healthier over time.
- Dick the Tramp - Never actually seen, this artifact (if that is even what it is) seems to cause noise and shaking inside the industrial plant in the Zone. A Nobel Prize winning scientist jokes that it could be a wind-up toy that a Visitor child accidentally left behind. Possibly the only long term 'inhabitant' of the Zone.
- Golden Sphere - Also known as the Wish Machine, this universally coveted artifact allegedly grants a wish of a person standing in front of it. It is a copper-colored sphere located behind a bulldozer at the entrance to a quarry inside the Zone. To reach it requires at least two people: The first person will be killed by a phenomenon called the "Meat Grinder" (located next to the bulldozer) which twists and crushes the person until what is left resembles ground meat. Killing someone in this fashion deactivates the Meat Grinder for some time. While the Meat Grinder is dormant, the second person can safely reach the Golden Sphere and make a wish. The Stalker known as Buzzard supposedly made multiple wishes that came true including wishing for a grown son and daughter. Redrick describes his impression of the Wish Machine as: "It lay at the foot of the quarry’s far wall, cosily resting amidst piles of rocks. It lay where it had fallen. Maybe it accidentally fell out of some monstrously huge pocket and got lost or rolled away during a game between giants. It had not been carefully placed here, it had been left behind, littering up the Zone like all the empties, bracelets, batteries, and other rubbish remaining after the Visitation."
- Lobster Eyes - Unknown function. Very rare item.
- Rattling Napkins - Unknown function. Very rare item.
- Ring - Very rare or possibly unique item. Redrick found one in the novel. It referred to have the size of an actual "ring", and once spun, it never stops, seemingly defying the first law of thermodynamics.
- Wriggling Magnet - Very rare or possibly unique item. Removed from the Zone by Stalker nicknamed Buzzard. Redrick suspects that Buzzard was granted a wish by the Golden Sphere to be able to safely retrieve and remove this unique item since it could not otherwise be reached without being killed. This theory is supported by the fact that Buzzard mentions to Redrick in the second act, "I've been to places you could only dream of". As Redrick was one of the most talented Stalkers, having explored most of the Zone, it would seem to imply that Buzzard used a wish to enter otherwise inaccessible areas of the Zone. Functionality unknown.
- Jolly Ghosts - A deadly, abnormal air turbulence that occurs in random parts of the Zone. Some Stalkers believe it to only be a legend but Redrick spots one from a safe distance in the last chapter.
- Witches Jelly - The scientists refer to this as a colloidal gas. The substance penetrates any organic material, plus plastic, metal and concrete. Only special ceramic vessels seem to contain it. Almost everything that it touches transforms into more Witches Jelly. It seems to collect in low-lying areas such as basements. At night it looks like alcohol burning with blue tongues. Apparently volatile, as Redrick mentions it "splashing out of the pit" in the garage on its own. Burbridge loses his legs to this in the second section.
- Greenie - A green colored substance that slithers like a long, thick snake randomly on the surface of the Zone. Possibly dangerous to humans, other properties unknown.
- Mosquito Mange - called gravi-concentrates by scientists. A spot within the Zone which exhibits extremely strong gravity, capable of crushing a person into a pancake or even pulling overhead helicopters violently to the ground. Stalkers search for mosquito manges by throwing small iron bolts ahead of them. If the bolt shoots into the ground at unnaturally high speed and great force, the spot is avoided. A stalker will use bolts to find a path around the Mosquito Mange.
- Replicas - Autonomous replicas of people buried in cemeteries inside the Zone before the Visitation. The replicas slowly shuffle about, seemingly possessing no intelligence but will return to the former residence of the deceased. Body parts of the replica are completely autonomous, and continue to function even if cut off. They move in a clumsy, jerky fashion after long pauses. Also known as Moulage.
- Silver Web - Encountered in the first act. Resembles a large spider web. It is invisible to some people. When the scientist Kirill backed into it, it made a "crackling" sound and vanished but he didn't see anything. Despite a close examination the web left no marks on Kiril, still hours later, he died of a heart attack. Though it is implied that the web may have killed Kirill like so many other phenomena in the zone it is impossible to establish for sure any connection.
- Spitting Devil's Cabbage - Never explained in the novel. Redrick mentions how the special suits are decent protection against it, so it may be inferred that this is some form of hostile alien plant that spits a harmful substance at anything that gets too close.
- Black Bramble - The black bramble supposedly indicates the Zone's border, so presumably this is another form of alien plant life.
- Cotton - Mysterious substance that tends to grow on metal, especially antennas. Some Stalkers aboard a helicopter once tried to retrieve an antenna with cotton growing on using a hook on a cable. The cable started smoking and "hissing poisonously", and the cotton started to grow up the cable.
- Burning Fluff - Some kind of irritating white fluff. For some reason, the wind never blows it out of the Zone. Redrick mentions how the special suits provide 100% protection against it, so it would seem to be much less threatening than some other Zone phenomena.
- Shadows - In several areas in the Zone, shadows are warped and twisted, in the opposite direction of where they should be. Buzzard claims that this phenomenon is "weird but harmless".
- Exploding Rainbows - Near the end of the novel, Redrick encounters a section of air "that shimmers and undulates, with hundreds of tiny rainbows exploding and dying". The phenomena's other properties are never explored, but because most anomalies in the Zones are dangerous, the party encountering it detours around it.
- Fire - An area of spontaneous combustion. It is unknown whether the spontaneous fires are triggered by the presence of people or not, but Redrick, the experienced Stalker, prefers to move away from anomalous fires, believing more may occur if he stays.
- Lightning - A form of pseudo-sentient lightning that originates from purplish-red dots. Found near the swamp leading to the quarry and the Golden Ball.
- Meat Grinder - a deadly anomaly. Outwardly, almost invisible. Anything that enters into the Meat Grinder's target area instantly twists, deforms and breaks into pieces, leaving behind only bloody smears. Only one stalker, Dixon, has ever survived the Meat Grinder, becoming a permanently deformed monstrosity. He always believed that Burbridge saved him. The Meat Grinder is located across the only path leading to the Golden Sphere. After the Meat Grinder is "triggered", it becomes inactive for a long time, allowing people walking behind the victim to safely pass through. The stalker Burbridge lured unsuspecting companions to their deaths by the Meat Grinder to reach the Golden Sphere alone.
- Shimmer - "Over the pile of old refuse, over broken glass and rags, crawled a shimmering, a trembling, sort of like hot air at noon over a tin roof. It crossed over the hillock and moved on and on toward us, right next to the pylon; it hovered for a second over the road - or did I just imagine it? - and slithered into the field, behind the bushes and the rotten fences, back there toward the automobile graveyard."
There are also a number of unexplained events or patterns associated with the Zone. Some relate to the Visitation itself. For example on the night of the Visitation many people were blinded, they all reported being blinded by the sound of a thunderbolt. But no one person who was not blinded heard the thunderbolt. Also the Stalker Red reports that people new to the Zone suddenly lose the ability to control their own speaking in the Zone, though this may be nerves or related to the Zone itself.