Portal stones are cylinder in shape and covered in symbols and characters. They are used to travel by someone using the One Power. By channeling into a certain symbol you can travel to another stone with corresponding symbols in this world and also other worlds. And possibly time travel as well. They were believed to have been made in the first age with the One Power. They cannot be moved as there is some force holding them in place.
In the Great Hunt Rand and some members of his party disappear while sleeping next to a Portal Stone and wake up next to a different one in another world. Where Loial remembers seeing a similar stone in an old book with the caption "From Stone to Stone run the lines of 'if,' between the worlds that might be." (TGH, Ch. 13). Rand later uses a Portal Stone and accidentally ends up four months into the future (TGH, Ch. 37).
The worlds through the Portal Stone are variants of the real world. Worlds that are simlar to the main world have a solid feeling, whereas worlds that diverge more strongly feel less substantial. Other worlds may contain a reflection of future events (TGS, Ch. 19). One constant is that the Dark One is imprisoned in all worlds. A second is that Tel'aran'rhiod lies within each of them (TGS, Ch. 21)
- Rhuidean - On the hillside above the city.
- Cairhien - Just outside the City
- Falme - At the base of the Toman Head peninsula
- Kinslayer's Dagger - In the foothills
- Tear - At least four, one not far from the city
- Other - One just a few miles south of the Shienar southern border
Rand travels to another world in his sleep (TGS, Ch. 13). The world is solid, but empty of animal life, other than Lanfear and packs of grolm. In this world, the Trollocs defeated Artur Hawkwing and they see a monument to that defeat (TGH, Ch. 16)
Rand returns to the real world through a portal stone (TGH, Ch. 17).
Rand takes people from Cairhien to Toman Head (TGH, Ch. 37).
Rand travels from Tear to the Waste (TSR, Ch. 22).
Those worlds truly are mirrors in a way, especially the ones where there are no people. Some of them reflect only great events in the true world, but some have a shadow of that reflection even before the event occurs. The passage of the Horn of Valere would certainly be a great event. Reflections of what will be are fainter than reflections of what is or what was, just as Hurin says the trail he followed was faint (Lanfear, (The Gathering Storm, Chapter 18).