Complex sentence: Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this led to its use by the Vikings to locate the Sun for navigational purposes.
Split into simpler sentences: Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance. This property has led to speculation that it was use by the Vikings to locate the Sun for navigational purposes.