The Lens itself is 18mm diameter with focal length of 2 inches or 50mm
  Make sure you place lens flat side down and convex side top.

In a nutshell there 3 most common types of mirrors used in laser cutters:

  Silicone (Si) with a thing gold layer on top.
    This mirror is relatively inexpensive and good for lasers up to 80 Watt. 
    It has great reflectance, but hard to clean and can get damaged easily.

  Molybdenum (Mo) polished to a mirror like surface.
    Very tough material, not easy to scratch and can be used in high powered lasers (over 80 Watt).
    Unfortunately it has somewhat low reflectance and costs more than Si.

  Polished Copper (Cu) mirror
      Has better reflectance than Mo, but less than Gold plated mirrors. 
      It’s also tought and good for high power lasers. 
      Unfortunately it’s difficult to find and probably expensive.

  K9 glass, gold coated
      This is one is not recommended for anything above 35Watt