The fortune stones were a group of small stones which the Ewok shaman Logray used to predict the future.
Logray was asked to use the stones by Chirpa after a mysterious light that fell from the sky (in actuality C-3PO and R2-D2 in an escape pod) to see if they were in danger or not. Their formation after falling onto the ground suggested that, on the day in question, a blinding flash of light will bring good fortune to them. Later on, Logray wondered if the fortune stones had gotten the events wrong.
It is speculated that the stones may have been infused with the Force, as they indeed gave the shaman the ability of foresight.