Palantíri.... the "seeing-stones" of The Lord of the Rings.
They were made by the Elves while they lived among the angelic Valar in the Blessed Land of Valinor in the West. Legend has it that they were made by Fëanor himself, the greatest of all Elvish craftsmen and magicians. How many were made, no one knows.
They came in various sizes, some only a foot wide. Others were massive objects 10 to 15 feet across. The largest palantír was the Master Stone, which could control all other stones. The Master Stone was kept in the topmost room of the tallest tower in the Elvish city of Avallónë on the island of Tol Eressëa (which was just off-shore from Valinor).
An unknown number of palantíri were given to the Númenoreans, the ancestors of the Men of Godnor. The Númenor collapsed into the sea as punishment for the Númenorean attempt to invade Valinor, all but seven were lost. These seven were taken by the Númenorean Elendil across the ocean to Middle-earth, where they became heirlooms of the new kingdom of Arnor and its lesser "pocket kingdom", Gondor. Arnor's capital was the great city of Annúminas, on the shorts of Lake Nenuial (about a day and half's ride north of the Shire).
Stone #1: Elostirion - Called "Elendil's Stone", it was used by Elendil to look west and see Tol Eressëa and the city of Avallónë. After his death at the hands of Sauron in the War of the Last Alliance, the Elvish king Gil-Galad built three towers in the Tower Hills (located only a day's ride west of the Shire). The tallest of these towers he called the Tower of Elostirion, and he put the stone in a secret room at the top of the tower. He aligned the palantír so it continually looked west at Avallónë.
Stone #2: Amon Sûl - This unnamed palantír was originally kept in the capital at Annúminas, but for most of its history it was in the great Watchtower of Amon Sûl. It was a massive stone, probably 10 feet across and second in size only to the one in Osgiliath. When Elendil's son, Isildur, died from an Orc arrow at the Disaster at Gladden Fields, his sons ruled in Arnor as High King. But the Gondorians were increasingly restive under their rule, and sought independence. When the tenth High King of Arnor died (some 860 years after Isildur), his two younger sons disputed their elder brother's right to rule. Arnor subsequently split into three kingdoms: Arthedain in the west and southwest; Cardolan in the south and center; and Rhudaur in the northeast. Annúminas was abandoned, and the capital moved a day's ride east to the new city of Fornost Erain on the North Downs. All three kingdoms claimed the palantír, so the Watchtower of Amon Sûl ("hill of the wind") was built where the three kingdoms came together, and the palantír installed there. 1,250 years later, the leader of the Nazgûl founded a realm for evil man just north and northeast of Rhudaur. He took the name Witch-king of Angmar, and after 110 years of war he destroyed Rhudaur and Cardolan. So many of the Dúnedain (the noble men of Arnor) died, Rhudaur's capital city was turned into a vast cemetery to bury them. This became a place of evil (the Barrow Downs), one that nearly led to the hobbits' death in "The Lord of the Rings". (Cardolan's southern capital city, Tharbad, was on the River Gwathlo. It survived for another 1,500 years but was destroyed by floods. Tolkien does not record the name of Rhudaur's capital.) Just before the Witch-king captured and destroyed the Watchtower, the palantír was taken to Fornost. It remained there for 465 years, until Angmar destroyed Arthedain as well. Arvedui, the king of Arthedain, fled into the northern wastes with the palantír. He and his party attempted to flee in a ship across the ice-locked Bay of Forochel in the dead of winter. Their ship was crushed in the ice, and the palantír losts beneath the waves.
Stone #3: Annúminas - This seeing-stone was also kept at Annúminas. It was taken to Fornost when Annúminas was abandoned. When Arvedui fled with the Stone of Amon Sûl, he also took the Stone of Annúminas with him. It, too, was lost beneath the cold waters of the Bay of Forochel. (It's probable the the Stone of Annúminas was small, only a foot across, and controlled by the Stone of Amon Sûl. That's why the three kingdoms didn't fight over it the way they did the Stone of Amon Sûl.)
Stone #4: Osgiliath - Osgiliath was the capital city of Gondor for most of its history. It straddled the River Anduin, and was considered the second-most beautiful city in Middle-earth (behind Annúminas). A great bridge was built over the Anduin in the center of Osgiliath. In the center of the bridge was a tower, and at the top of the tower a domed room whose ceiling was painted to resemble the night sky. Here was placed the exceptionally massive Stone of Osgiliath, some 15 feet across. This palantír had a special power: While only two palantíri could communicate with one another at the same time, the Stone of Osgiliath could eavesdrop on this communication. Twenty-eight years after Rhudaur and Cardolan fell, Gondor was wracked by civil war. Castamir, the head of Gondor's navy, overthrew the rightful king, Eldacar. Castamir's assault on Osgiliath caused the city to burn. The bridge collapsed, and the Stone of Osgiliath was lost in the Anduin. (190 years later, Osgilith was devastated by plague. It was abandoned, and the capital moved to Minas Tirith.)
Stone #5: Minas Ithil (Minas Morgul) - Some 1,350 years after Elendil founded Arnor and Gondor, his successors founded the city of Minas Ithil ("Tower of the Moon"). This was supposed to be the second great city of Gondor. The Ephel Duath was the mountain range which protected Mordor's western boundary. A spur of the mountain range jutted westward into the Gondorian province of Ithilien. On the southern side of this spur was valley. At the head of the valley was built the delicate, ethereal city of Minas Ithil. A stream (the Morgulduin) formed the valley's southern boundary, and to access the city one had to cross a white bridge. Minas Ithil was made to guard the Cleft of Cirith Ungol, which was the only pass through the Ephel Duath into Mordor. To reach the Cleft of Cirith Ungol, one took the Morgul Road -- which traveled along a cleft in the spur and east to the pass. A footpath outside the city ascended a steep set of rough stairs to reach the ridge atop the spur. This path then led through tunnels to the Cleft. Minas Ithil was built of white stone. In the center of the city was a huge tower, the Tower of the Moon, which was crowned with the image of a waning moon. The fifth palantír was placed in a secret room at the top of the Tower of the Moon. Unfortunately, just 120 years after the founding of Minas Ithil, the War of the Last Alliance occurred. The city was captured and its population wiped out. The palantír, thank goodness, was not discovered, and the city recaptured after four years. The Sauron's forces were defeated seven years later when Isildur cut the One Ring from Sauron's hand, and Minas Ithil recaptured. Gondor decided to reinforce Minas Ithil by building a watchtower -- the Tower of Cirith Ungol -- astride the footpath to help guard the Cleft and prevent the city from being surprised again. Two thousand years later, Minas Ithil fell to the Nazgûl and the Ithil-Stone came into Sauron's hands. (Minas Ithil was renamed Minas Morgul -- the "Tower of Black Sorcery".) Sauron moved the palantír to the Tower of Barad-dûr. It is still buried in the wreckage of the Dark Tower.
Stone #6: Orthanc - A year after the founding of Arnor and Gondor, Elendil faced a problem. While Arnor ruled most of the vast plains between the Misty Mountains in the east and the Blue Mountains in the west, it didn't rule the foothills of the Misty Mountains. These were occupied by the Dunlendings, a violent and foul people. At the time, Gondor was also much larger -- and controlled what later became Rohan. Thus, between the Gap of Rohan and the River Gwathlo was a swath of land controlled by a bloody nation that loved to raid the much wealthier Dúnedain. Elendil decided to build two watchtowers on either side of the Gap. The one in the south was Aglarond, built at the head of a deep valley. (This was later given to the Rohirrim, who named the valley Helm's Deep and the fortress Súthburg -- and, later, the Hornburg.) The fortress in the north was named Angrenost (Isengard). Also built in a valley (from which issued the River Isen), it was a vast area encircled by a high stone wall. In the center of the fortress was the tower Orthanc, 400 feet tall and riddled with secret passageways and rooms. A whopping 4,500 years later, the wizard Saruman asked the Steward of Gondor for the keys to Orthanc. The Steward gladly turned them over; Gondor lacked the men and resources to keep Angrenost staffed, but did not want to merely abandon it. By this time, the palantír of Orthanc had largely been forgotten; even the chamber in which it resided was lost to memory. Saruman searched for years for the palantír, eventually locating it. He used it to spy on his neighbors and enemies, and later used it to try to locate the One Ring. When Saurman was defeated for the first time during the War of the Ring, Gríma Wormtongue cast the palantír from Orthanc, where it was recovered by Gandalf. Gandalf turned the stone over to Aragorn, and it was kept at Minas Tirith.
Stone #7: Minas Tirith (Minas Arnor) - Minas Arnor ("Tower of the Setting Sun") was the third great city of Gondor. It was originally a garrison town, designed to stop an army from coming up from the south, west from Rohan, or coming down from the north from Rhovanion. A giant spur like a ship's prow jutted from the mountain. Seven levels in the shape of set-back concentric rings formed the city, which was literally carved from the mountain. At the top, level with the highest part of the prow, was built the gleaming White Tower. It's not clear where the palantír was stored in Minas Arnor. But some 3,400 years after Minas Arnor's founding, the White Tower was rebuilt. This took 200 years! Afterward, the palantír was placed in a secret chamber at the very top of the tower. Only the King of Gondor, and, later, the Steward, knew where the chamber was and how to access it. Just 65 years later, however, Minas Ithil was lost to Sauron. Minas Arnor was renamed Minas Tirith ("Tower of Guard"). The King stopped using the palantír, for fear that Sauron would align his seeing-stone with it and spy on Gondor or attempt to influence the King through the stone. 650 years later, the White Tower was rebuilt again. This reconstruction, by the Steward Ecthelion, took just 13 years, and again the palantír was placed in the topmost secret chamber (but remained unused). The White Tower was renamed the Tower of Ecthelion in his honor, and the palantír came to be known as the Ecthelion-stone. Sixteen years before Bilbo's 111th birthday, Denethor became Steward of Gondor. Ignoring the warnings of his predecessors, Denethor began using the palantír to learn about Sauron's preparations and movements and better protect his city. But Denethor did not realize that Sauron was sutbley influencing him through the palantír, and Denethor began to age quickly and his mind turned to bitterness and despair. Denethor was holding the seeing-stone when he committed suicide on a funeral pyre in Rath Dínen. (The movies depict him afire, leaping over a cliff. The novel does not; Denethor dies on his funeral pyre in the Streets of Silence.) The palantír was rendered virtually unusable by the fire. Most people could now only see two flaming hands if they looked into the stone. Only the King and a handful of powerful, strong-willed others could use the Ecthelion-stone to see and hear any distance or communicate with another palantír. (Presumably, the Ecthelion-stone and the Orthanc-stone were both placed back in the secret chamber atop the White Tower.)
Most of these images are from the Peter Jackson movies, but the one painting is by the Brothers Hildebrandt.