Unidentified UNSC medal
From Halopedia, the Halo wiki
The Unknown UNSC Medal is a decoration/medal awarded to Soldiers, Sailors, and Marines in the UNSC Defense Force. It is rumored to be the Colonial Cross, one of the highest decorations in the UNSC. However, due to the fact it is not shaped like a cross, it is unlikely, it may however be a Human-Covenant war Campaign medal.
It has recently been seen in some of the Halo 3 Advertisements.