I see a texture on the icing but no diamond impression. That looks like bc or royal icing dots that were piped on or fondant cut out with a small round tip.
I would think you could use some kind of screen (food-safe, of course) rather than an impression mat. And then as others have said, pipe on top of that.
I see what you are talking about and wonder if it isn't a piece of metal impression mat like used in clay making products. When I was looking at AC Moore this weekend, I was in the clay aisle and saw some metal sheets (like maybe 4x6) that had different designs. Maybe the scale texture in this one:
Hope that helps
yummy -the impression is in the fondant on the top and botton tiers
ptanyer -I will go take a look this week and see if they will work
anyone else have an idea how to make is very small impression?