The rail rang and snapped. The stair cracked and splintered in glittering sparks. But the ball was unharmed: it rolled on down the steps, a globe of crystal, dark, but glowing with a heart of fire. As it bounded away towards a pool, Pippin ran after it and picked it up.

(LoTR, III-10)

He drew his cloak aside and gazed at it. The air seemed still and tense about him. At first the globe was dark, black as jet, with the moonlight gleaming on its surface. Then there came a faint glow and stir in the heart of it, and it held his eyes, so that now he could not look away. Soon all the inside seemed on fire; the ball was spinning, or the lights within were revolving. Suddenly the lights went out. He gave a gasp and struggled; but he remained bent, clasping the ball with both hands. Closer and closer he bent, and then became rigid; his lips moved soundlessly for a while. Then with a strangled cry he fell back and lay still.

(LoTR, III-11)

The "lore of the Stones" is now forgotten, and can only be partly recovered by conjecture and from things recorded about them. They were perfect spheres, appearing when at rest to be made of solid glass or crystal deep black in hue. At smallest they were about a foot in diameter, but some, certainly the Stones of Osgiliath and Amon Sûl, were much larger and could not be lifted by one man. Originally they were placed in sites suitable to their sizes and intended uses. standing on low round tables of black marble in a central cup or depression, in which they could at need be revolved by hand. They were very heavy but perfectly smooth, and would suffer no damage if by accident or malice they were unseated and rolled off their tables. They were indeed unbreakable by any violence then controlled by men though some believed that great heat, such as that of Orodruin, might shatter them, and surmised that this had been the fate of the Ithil-stone in the fall of Barad-dûr.
Though without any external markings of any kind they had permanent poles, and were originally so placed in their sites that they stood "upright:" their diameters from pole to pole pointed to the earth's centre, but the permanent nether pole must then be at the bottom. The faces along the circumference in this position were the viewing faces, receiving the visions from the outside, but transmitting them to the eye of a "surveyor" upon the far side. A surveyor, therefore, who wished to look west would place himself on the east side of the Stone, and if he wished to shift his vision northward must move to his left, southward. But the minor Stones, those of Orthanc, Ithil, and Anor, and probably Annuminas, had also fixed orientation in their original situation, so that (for example) their west face would only look west and turned in other directions was blank. If a Stone became unseated or disturbed it could be re-set by observation, and it was then useful to revolve it. But when removed and cast down, as was the Orthanc-stone, it was not so easy to set right. So it was "by chance" as Men call it (as Gandalf would have said) that Peregrin, fumbling with the Stone, must have set it on the ground more or less "upright," and sitting westward of it have had the fixed east-looking face in the proper position. The major Stones were not so fixed: their circumference could be revolved and they could still "see" in any direction. [17]
Alone the palantíri could only "see:" they did not transmit sound. Ungoverned by a directing mind they were wayward, and their "visions" were (apparently at least) haphazard. From a high place their westward face, for instance, would look to vast distance, its vision blurred and distorted to either side and above and below, and its foreground obscured by things behind receding in ever-diminishing clarity. Also, what they "saw" was directed or hindered by chance, by darkness, or by "shrouding" (see below). The vision of the palantíri was not "blinded" or "occluded" by physical obstacles, but only by darkness; so they could look through a mountain as they could look through a patch of dark or shadow, but see nothing within that did not receive some light. They could see through walls but see nothing within rooms, caves, or vaults unless some light fell on it; and they could not themselves provide or project light. It was possible to guard against their sight by the process called "shrouding," by which certain things or areas would be seen in a Stone only as a shadow or a deep mist. How this was done (by those aware of the Stones and the possibility of being watched by them) is one of the lost mysteries of the palantíri. [18]
A viewer could by his will cause the vision of the Stone to concentrate on some point, on or near its direct line. [19] The uncontrolled "visions" were small, especially in the minor Stones, though they were much larger to the eye of a beholder who placed himself at some distance from the surface of the palantír (about three feet at best). But controlled by the will of a skilled and strong surveyor, remoter things could be enlarged, brought as it were nearer and clearer, while their background was almost suppressed. Thus a man at a considerable distance might be seen as a tiny figure, half an inch high, difficult to pick oat against a landscape or a concourse of other men; but concentration could enlarge and clarify the vision till he was seen in clear if reduced detail like a picture apparently a foot or more in height, and recognized if he was known to the surveyor. Great concentration might even enlarge some detail that interested the surveyor, so that it could be seen (for instance) if he had a ring on his hand.
But this "concentration" was very tiring and might become exhausting. Consequently it was only undertaken when information was urgently desired, and chance (aided by other information maybe) enabled the surveyor to pick out items (significant for him and his immediate concern) from the welter of the Stone's visions. For example, Denethor sitting before the Anor-stone anxious about Rohan, and deciding whether or not at once to order the kindling of the beacons and the sending out of the "arrow," might place himself in a direct line looking north-west by west through Rohan, passing close to Edoras and on towards the Fords of Isen. At that time there might be visible movements of men in that line. If so, he could concentrate on (say) a group, see them as Riders, and finally discover some figure known to him: Gandalf, for instance, riding with the reinforcements to Helm's Deep, and suddenly breaking away and racing northwards. [20]
The palantíri could not themselves survey men's minds, at unawares or unwilling; for the transference of thought depended on the wills of the user on either side, and thought (received as speech) [21] was only transmittable by one Stone to another in accord.

[17] A later, detached note denies that the palantíri were polarized oriented, but gives no further detail.

[18] The later note referred to in note [17] treats some of these aspects the palantíri slightly differently; in particular the concept "shrouding" seems differently employed. This note, very hasty and somewhat obscure, reads in part: "They retained the images received, so that each contained within itself a multiplicity of images and scenes, some from a remote past. They could not 'see' in the dark; that is, things that were in the dark were not recorded by them. They themselves could be and usually were kept in the dark, because it was much easier then to see the scenes that they presented, and as the centuries passed to limit their 'overcrowding.' How they were thus 'shrouded' was kept secret and so is now unknown. They were not 'blinded' by physical obstacles, as a wall, a hill, or a wood, so long as the distant objects were themselves in light. It was said, or guessed, by later commentators that the Stones were placed in their original sites in spherical cases that were locked to prevent their misuse by the unauthorized; but that this casing also performed the office of shrouding them and making them quiescent. The cases must therefore have been made of some metal or other substance not now known."
Marginal jottings associated with this note are partly illegible, but so much can be made out, that the remoter the past the clearer the view, while for distant viewing there was a "proper distance," varying with the Stones, at which distant objects were clearer. The greater palantíri could look much further than the lesser; for the lesser the "proper distance" was of the order of five hundred miles, as between the Orthanc-stone and that of Anor. "Ithil was too near, but was largely used for [illegible words], not for personal contacts with Minas Anor."

[19] The orientation was not, of course, divided into separate "quarters" but continuous; so that its direct line of vision to a surveyor sitting south-east would be to the north-west, and so on. [Author's note.]

[20] See The Two Towers III 7.

[21] In a detached note this aspect is more explicitly described: "Two persons, each using a Stone 'in accord' with the other, could converse, but not by sound, which the Stones did not transmit. Looking one at the other they would exchange 'thought' – not their full or true thought, or their intentions, but 'silent speech,' the thoughts they wished to transmit (already formalized in linguistic form in their minds or actually spoken aloud), which would be received by their respondents and of course immediately transformed into 'speech,' and only reportable as such."