From Halopedia, the Halo wiki
The Unknown UNSC Medal is a decoration/medal given to Soldiers, Sailors, Marines, and Airmen in the UNSC Defense Force. It is rumored to be similar in function to the Colonial Cross, one of the highest decorations in the UNSC. It may be for serving in the Human-Covenant War Campaign medal.
It has been seen in some of the Halo 3 advertisements. The medal is shown being gold with the UNSCDF insignia on it.