Co2 doesn't emit at 1090nm bud. Did you mean to say 10900nm as that would be more accurate. Standard emission of co2 is at 10800nm so pretty much nothing has that wl. Therefore, nothing can really reflect it.
Rest of the info is solid
some small corrections. Co2 lasers emit at 10,600nm. mirrors for co2 lasers can be made from several different things such as copper, molybdenum, or silicon. sometimes mirrors are gold plated.
you actually have to be very careful about what kind of metals you're engraving. unexpected things become opaque, transmissive, or reflective at this wavelength.
i built this laser cutter. it has three mirrors that are used to deliver the IR from the tube through the focusing lens in the head and to the material being engraved or cut.