There have been some questions recently regarding the allowing (or not) of jewellery. To clarify: jewellery is NOT permitted (even if covered with tape). This includes piercings.
This ruling comes down from the governing body of all soccer: FIFA. The specific rule is noted below.
Law 4 - The Players' Equipment
A player must not use equipment or wear anything that is dangerous.
All items of jewellery (necklaces, rings, bracelets, earrings, leather bands, rubber bands, etc.) are forbidden and must be removed. Using tape to cover jewellery is not permitted.
The players must be inspected before the start of the match and substitutes before they enter the field of play. If a player is wearing or using unauthorised/ dangerous equipment or jewellery the referee must order the player to:
A player who refuses to comply or wears the item again must be cautioned.