Guidance Stones are items from The Legend of Zelda: Breath of the Wild. There are several Guidance Stones found within certain Shrines, in every last Sheikah Towers, the Hateno Ancient Tech Lab, and the four Divine Beasts. They have varying functions depending on where they are located, and they all require the Sheikah Slate to function.
Within the Sheikah Towers, their purpose is to unveil each regional map of Hyrule with data on the Sheikah Slate. Once the Guidance Stone is used and the tower is activated, Link can warp back to the tower at a moment's notice. If Link can activate a second Tower, he will earn the Sheikah Sensor.
Within the Hateno Ancient Tech Lab, Purah happens to own a Guidance Stone herself. However, when Link first arrives to the Lab, it is shown that the Guidance Stone is inert as the blue fire that gives it power has gone out. Link must then search for the Ancient Furnace to acquire the blue flame using a torch, then must take the flame uphill while lighting the lanterns along the way to serve as a checkpoint in case his Torch goes out and finally, reignite the stove that fuels the Lab's Guidance Stone so that Link can obtain the Camera Rune, Hyrule Compendium, as well as restore photos taken by Zelda that that have a role in helping the restoration of Link's memories.
After taking a snapshot of Purah, she will then allow Link to upgrade two Runes and the Sheikah Sensor in exchange for some Ancient materials, these being three Ancient Cores for the Stasis Rune, three Ancient Shafts for the Remote Bombs, and finally three Ancient Screws to upgrade the Sheikah Sensor.
The Ancient Oven
In addition to normal Guidance Stones, during the Age of Burning Fields the Sheikah technology researcher Robbie modified his Guidance Stone at the Akkala Ancient Tech Lab to create the Ancient Oven Cherry which could convert guardian parts into ancient soldier gear. Later on, Robbie attempted to modify Cherry even further though his plans where too complex to implement and decided to put them on hold. However his wife Jerrin who had originally been sent by Impa to aid Robbie in his research, managed to complete Robbie's planned upgrades, allowing Cherry to speak like a living person. However Jerrin eventually became jealous of Robbie's interactions with Cherry as he came to rely on it more and more, which Robbie attributed to jealousy towards Cherry's namesake as he had named the Ancient Oven after his first love. After learning of his wife's feelings, Robbie chose to restore Cherry back to her original mechanical way of speaking and never referred to her as Cherry in order to avoid upsetting his wife.
Eventually the blue flame that powered Cherry went out and neither Robbie nor Jerrin had been able to relight it before Link's arrival. After proving his identity to Robbie, he is asked to relight the furnace that powers Cherry so she can make Link ancient soldier gear. Like with Purah's guidance stone, Link must relight the furnace with Blue Flame which he can obtain from the Ancient Furnace in Deep Akkala. After restoring power to Cherry, Link is allowed to use her to forge ancient soldier gear in exchange for Rupees and Guardian Parts.