LONDON (AP) — Researchers say a rough, whitish crystal recovered from the wreckage of 16th century English warship may be a sunstone, a special kind of mineral believed by some to have helped medieval seafarers navigate the high seas.

Historians have long wondered at the ability of Vikings explorers to travel long stretches of open water. Some say they used transparent calcite crystals — whose properties can allow a skilled sailor to guess the position of the sun during overcast or foggy days — as navigational aids.

One Icelandic legend refers to a “sunstone” used in such a way, but none has ever been discovered in a seafaring context.

French researchers said Thursday they identified one taken from the bottom of the English Channel, potentially lending substance to the legend.