What Tree is That? The National Arbor Day Foundation's version of a tree identification tool. A series of pictures and questions are used like a dichotomous key to identify mystery trees by process of elimination. A very good animated interactive tutorial illustrates how to use a dichotomous key.
Osmium twinning / Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family and is the densest naturally occurring element, with a density of 22.59 g/cm3 (slightly greater than that of iridium and twice that of lead).