The problem here is context. Without any archaeological context - such as a terminus post or ante quem level - it's very hard to ascribe a meaningful significance: The French took a colonial interest in Canada from the 1520s onwards, following the English expedition of 1497. This already lies within the potential age range of the artefact. It is hardly inconceivable that a French ship of the period landed on or near Oak Island. Let us suppose for the sake of argument, however, that the cross does date from 12-1300; without a reliable context to show that it was deposited at that time, that information is far less useful. The object could just as well have been carried to the island in 1700 or 1980.