Inspired by similar Fresnel experiments floating around the net, I decided to try melting a penny. On winter solstice, I found that a zinc penny melts within a minute when held in the focus. Solid copper pennies (from 1982 or earlier) wouldn't melt, but probably would during summer. Copper's melting point is almost 2000oF compared to Zinc's 790oF. See the first row of images for these tests.
With the Fresnel lens oriented correctly, I had another crack at melting those coins. The following video and the second row of images shows my results. MUHAHAHAHA!!!
Note: Copper's melting point is about 2000oF, but Nickel's is 2600o. So it's highly possible that only the copper in the coin (75% copper, 25% nickel) melted, resulting in the mutilated pitted surface.