Things you cannot see in a meteorite
People contact me frequently saying that their rock must be a meteorite because they can “see” diamonds, lonsdaleite, carbonado, gold, iridium, platinum, or other rare things. You cannot see and identify any of these things by sight in a meteorite.
Mineral phases
Diamonds, lonsdaleite, and carbonados occur in some meteorites but they are all microscopically small and can only be seen with a microscope or other expensive instruments like a scanning electron microscope (SEM). More importantly, even if you could see them it is impossible to identify a diamond, lonsdaleite, or carbonado by sight. Again, expensive instruments are required. Just because it is shiny or black does not mean that it is a diamond or carbonado. There are many other kinds of shiny and black minerals that are seen in earth rocks. See also lonsdaleite diamonds.
Rare chemical elements
Certain chemical elements are much more abundant in meteorites (especially chondrites, i.e., 93% of stony meteorites) than in typical rocks of the earth’s crust. Geochemists call these elements siderophile (iron loving) because in meteorites they occur mainly in iron-nickel metal (but also some sulfide minerals). These elements include nickel, cobalt, and 8 highly siderophile elements. The HSEs, in decreasing order of abundance, are platinum, ruthenium, palladium, osmium, iridium, rhodium, gold, and rhenium. Note that all of these elements occur in concentrations of less than 2 ppm (<2000 ppb) in ordinary chondrites (Table). Concentrations are even lower in other types of stony meteorites. This low abundance means that you cannot see highly siderophile elements in a meteorite. You might be able to see grains of Fe-Ni metal, but without chemical analysis you cannot conclude that the metal contains any of the highly siderophile elements. Note that the relative abundance of the highly siderophile elements is very similar in all ordinary chondrites, e.g., the ratio of Ir to Au (Ir/Au) is typically 2.5 to 3.5.
As I note elsewhere, hand-held X-ray fluorescence (XRF) analyzers often report the presence of highly siderophile elements in stony meteorites but such reports are always erroneous in that the analyzers cannot “see” any element that occur at such low concentrations.