Foil art. We then used spray adhesive and laid foil over the top. They pressed the foil down around their yarn pieces to create that embossed look. Lastly, they used colored Sharpies to color the recessed areas and left the embossed "lines" silver.
From dickblick.com. I totally want to try this using "matte black metal sheet." Apparently the relief work is done with colored pencils. I've done repousse and chase projects with copper metal and a higher gauge aluminum metal, but embossing with color looks cool.