Cold welding and graphite lubricant failure in early space missions

Diamond is very hard, very strong bonds. Graphite is a planar structure — these things slip, slide over each other. When you get to cold welding — you'll hear me tell the story about the first initial space shots and how graphite didn't work as a lubricant. So the crystal structures, diamond cubic versus hexagonal. Density is significantly different. Optically, diamonds are transparent to visible light, graphite's black. Why? This structure leaves me some free electrons which absorb light. The diamond structure leaves me no free electrons. Because of that, diamond's an insulator and graphite's a conductor. There are 12 to 15 orders of magnitude difference in conductivity of diamond versus graphite.