Klaxas: A spell may have a single subtype, multiple subtypes, or no subtype at all. In the case of Moonglow Amulet, it is actually supposed to be of the Mana subtype - good catch spotting that! If other cards have no subtype printed, it is not missing from the card - it simply was meant to have no subtype.
stadi: All cards are considered spells, including equipment.
Thanks guys!