From the US Navy grooming standards instruction
6. JEWELRY. Conservative jewelry is authorized for all personnel and shall be in good taste while in uniform. Eccentricities or faddishness are not permitted. Jewelry shall not present a safety or FOD (Foreign Object Damage) hazard. Jewelry shall be worn within the following guidelines:
a. Rings. While in uniform, only one ring per hand is authorized, plus a wedding/engagement ring set. Rings are not authorized for wear on thumbs.
b. Earrings
(1) Men. Not authorized while in uniform. Additionally, earrings are not authorized in civilian attire when in a duty status or while in/aboard any ship, craft, aircraft, or in any military vehicle or within any base or other place under military jurisdiction, or while participating in any organized military recreational activities. When considered appropriate by the prescribing authority under article 7201.2, earrings may be prohibited while in foreign countries.
(2) Women. One earring per ear (centered on earlobe) may be worn while in uniform. Earrings shall be 4mm - 6mm ball (approximately 1/8 - 1/4 inch), plain with shiny or brushed matte finish, screw on or with posts. Gold for officers/CPOs, and silver for enlisted personnel. Small single pearl earrings are authorized for wear with Dinner and Formal Dress uniforms.
c. Body Piercing. Not authorized while in uniform. No articles, other than earrings for women specified above, shall be attached to or through the ear, nose, or any other body part. Additionally, body piercing is not authorized in civilian attire when in a duty status or while in/aboard any ship, craft, aircraft, or in any military vehicle or within any base or other place under military jurisdiction, or while participating in any organized military recreational activities. When considered appropriate by the prescribing authority under <article 7201.2>, body piercing may be prohibited while in foreign countries.