I have compiled a list of sources for the elements that are available to the amateur chemist. Iridium will be discussed here.
Iridium is one of the rarest and densest elements in the earth's crust. It is found in small concentrations in platinum ores and competes with osmium for position of densest element, the two's densities being extremely close (22.56 grams per cubic centimeter for iridium, compared with 22.59 for osmium). Iridium's is named after the "goddess of the rainbow" because of the many colors that it exhibits in its salts. Meteorites tend to contain larger quantities of iridium than the general earth. Iridium does not dissolve in any acids, but is attacked by halogens and some molten salts when heated to a high temperature. Iridium black is an extremely black pigment for ceramics made of finely divided iridium metal. See this link for more information about the colorfulness of iridium chloride and its complexes.
In element form: Iridium spark plugs often contain pure iridium or iridium-platinum alloys plated on the central electrodes. Osmiridium pen tips for expensive pens contain iridium metal.
In compound form: No sources found.
Here is my sample of iridium. It is a spark plug with the electrode coated with a platinum-iridium alloy (90% Pt 10% Ir is typical).