Sending Stones

The Sending Stones are anchors in time that allow the The Navigators move between historical periods. They can be anything: an ancient circle of stones, a man-made building, a rock on a mountain... There are a lot of theories on what can make a Sending Stone: some think it could be only silicon based, others assume that it could be anything almost non-movable, and if it were moved, the Stone would deactivate. Most think it's a random phenomenon, or a result of a divine intervention, or that they are of magical nature. Truth is, as with many Fringe things, Sending Stones are vastly unresearched.

Manifestation

For ordinary people, and even for most Fringers, there is no telling if something is a Sending Stone or not. It can only be detected by Huncher Navigators, and just recently the Analytical Navigators learned how to feel them without using technology, though the margin of mistake is still very high.
Huncher Navigators see the Stones very differently, in a sense. Not visually, they still look like normal things, but around them there is something described as an emotional field of some kind. Different Navigators feel different things, which actually can determine a lot in their ability to move through the Time Current. Some experience uneasiness, some - unsolicited joy, or excitement of an imminent adventure. Damien Sinclair feels the desire of risk, for example.

Localization

The Stones adhere to rules, most of which are, of course, unknown. But there are two which have pretty solid emphyric evidence to be considered full fledged rules.
First, the Stone always brings you to itself. They are like bridges, in a way that they do not hurl you into the Time Stream and let you drop out in the same relative point in space, but they also bring you to their exact position in the time period you're going to, barring some margin for mistakes.
And the second rule is that the Stones have periods of activity, which are yet to be calculated, but possible to be determined while the stone is active. A Stone has a range of time it is active. If it works starting November 11, 1444 up until January 1, 1821, anybody from any moment in history inside this range can use the stone to travel back in time up to the first day of its activity. Time-travel to the future isn't possible without a Navigator from the future, of course. As soon as the Stone becomes inactive, Navigators stop feeling it, when they're close to it.
Nobody knows what defines that range of activity, but local events apparently play the part somehow. The most famous Stone in the world - one of the walls of Notre Dame de Paris - was active almost a thousand years, and it stopped working on April 15, 2019, the day of the dreadful fire.

Type

Metaphysical, Supernatural

Theory of Convergence

There aren't many theories about the Sending Stones, since there aren't that many Time Travelers around, but one thing really sticks out about them. They're unevenly distributed throughout the world. Like modern historical science itself, they tend to converge in Europe and the Middle East, with fewer numbers in other parts of Asia and Africa, and a very small number in the Americas. There aren't any known ones in Australia and Antarctica.
If one looks at the map of known Sending Stones, one would see that the biggest number of them can be found in Pannonia, with the biggest concentration of them found between Bratislava, Budapest and Lviv. Then, going west and going east, there is less and less of them. Though the most rational explanation for that is Europe being way more thorough in researching and finding the Stones, many Fringers believe there must be some kind of explanation of magical or celestial nature.
Navigators-analysts are more interested in activating Stones. Already by the middle of the "zero" years, this small community discovered the presence of special auras around these points of entry into the Time stream, but they have not yet deciphered their meaning. Studying the nature of the Stones requires active experimental time travel, and this, in turn, requires great sacrifice. The Navigator-Analyst community is not yet big enough to be prepared for it. Therefore, "breakthroughs" in the science of studying Stones are quite rare. The next discovery after auras was that sleeping Stones, in theory, can be activated during periods when they do not work. And Remy Dalton discovered in 2020 that stones can work not only within one large period, but also in smaller, scattered intervals along the timeline. He also managed to activate the dormant (but almost awakened) Stone ahead of time, "feeding" him magical energy through a makeshift funnel. Where this thought came from for Remy is unclear, but the fortunate coincidences in these events hint at outside interference.