Were the Palantiri truly powerful objects?

[Odin]

How powerful were the Palantiri?

Some sources say that they were seeing-stones, able to view people and locations from great distances. And yet other sources only refer to them as communication devices, able to contact other Palantiri.

How powerful were the Palantiri?

 

[Mahanaxar]

Well, think of the Palantiri as a Middle Earth communication devices between two distant places, hence their name Palantiri (Stones of Seeing).

Elendil son of Amandil, father of Isildur and Anarion brought them to Middle Earth.

As far as I know there are 7 of them :

In the Northern Kingdom:
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1. Elendil Stone: The Seeing-stone of Emyn Beraid was housed in the Elf-tower of Elositirion among the Tower Hills, west of the Shire. This palantír was aligned westwards, and could be used to see along the length of the Straight Road to the shores of the Blessed Realm.

2. Stone of Amun Sul: The largest of the three palantíri of Arnor, and said to be the most powerful, was originally housed in the Tower of Amon Sul in the center of the North-kingdom. When the Tower was destroyed by the Witch-king of Angmar, the stone was rescued and taken to Fornost. When Fornost itself came under attack, King Arvedui escaped into the north with this palantír and the Stone of Annúminas. Both stones were lost when Arvedui's ship sank in the Icebay of Forochel.

3. Stone of Annuminas: This palantír was nonetheless the one most used by the Kings of Arnor and Arthedain. It was originally held in Elendil's capital at Annúminas, but when the house of the Kings was moved to Fornost, the stone was moved, too. And was lost with the Stone of Amon Súl.


In the southern Kingdom:
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1. The Master Stone: The Master Stone of Middle Earth was held in Gondor, beneath the Dome of Stars. After the siege of Osgiliath, the city was ruined and the palantír was lost in the Anduin.

2. Stone of Anor: Also called the Stone of Minas Tirith, the palantír held at Anárion's stronghold of Minas Anor remained there throughout the Third Age. Denethor used it in secret, but Sauron subverted it and drove the Steward to madness. Denethor burned himself to death with the palantír in his hands, and from that time on the image of the burning hands of the Steward was fixed within the stone.

3. Stone of Ithil: The counterpart of the Stone of Anor, held in Isildur's city of Minas Ithil beneath the mountains of the Ephel Dúath in Ithilien. In the third age, the Nazgul assaulted Minas Ithil and captured the city along with the stone. Thus Sauron was later able to use the Ithil-stone to influence both Saruman and Denethor in the War of the Ring. The fate of this palantír is not known but I think it was destroyed in the Downfall of Barad-dûr.

4. Stone of Orthanc: Held in the tower of Orthanc in Isengard, which was originally a northerly outpost of Gondor. Saruman later petitioned Steward Beren of Gondor to grant him the Key of Orthanc, in hope that he would find the stone still within the tower. His hope was fulfilled, but through the stone he was ensnared by Sauron. After the fall of Isengard it came to the hands of Aragorn, who as the true heir of the Kingship of Gondor was its rightful keeper.


So yes, the Palantiri had a good deal of power in them, I don't know how to specifically answer your question, but I don' think they had any powers other than sightseeing and communication, the foulest use for them was in what Sauron did but that's about it.

 

[Valandil]

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3. Stone of Ithil: The counterpart of the Stone of Anor, held in Isildur's city of Minas Ithil beneath the mountains of the Ephel Dúath in Ithilien. In the third age, the Nazgul assaulted Minas Ithil and captured the city along with the stone. Thus Sauron was later able to use the Ithil-stone to influence both Saruman and Denethor in the War of the Ring. The fate of this palantír is not known but I think it was destroyed in the Downfall of Barad-dûr.
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So yes, the Palantiri had a good deal of power in them, I don't know how to specifically answer your question, but I don' think they had any powers other than sightseeing and communication, the foulest use for them was in what Sauron did but that's about it.

I thought the implication was that the Ithil Stone had been taken to Sauron - and that this was how he 'trapped' both Saruman and Denethor, through their respective uses of the Orthanc and Anor Stones.

From a statement of Gandalf's, I think that the Stones could also show things across time - at least from the past. They were 'seeing-stones' - and I think they retained a memory of all that they saw.