The insignia of non-commissioned UNSC Marines has never been revealed. The only description we have of Marine insignia is from Halo: Contact Harvest, page 47:
"...the Marine Corps Staff Sergeant indicated by the four gold chevrons on his filthy dress coat's sleeve."
This lead some to believe that UNSC Marine insignia was identical to that of the United States Marine Corps, a reasonable assumption, considering that they share the same ranks and organization. However, with no other information, the insignia could just as likely be the same as a British Royal Marines Colour Sergeant (Check the page for Royal Navy ratings rank insignia. It's there).
But, with the protagonist of Halo 3: ODST being a Lance Corporal, this picture caught my attention:
Halo 3: Recon Screenshot - Upper right chest plate has sign of one chevron and two crossed rifles
Is this proof of UNSC and US Marine insignia being identical? Your comments, please.