Signet rings are frequently decorated with intaglio, as are seals . An example of an investment compound is plaster of paris mixed with silica, boric acid, and graphite. Iolite is not rare and has a hardness of 7 - 7.5. Iolite is found in Sri Lanka, India, Madagascar and Burma. An iridescent object displays many lustrous, changing colors.
Iridescence is caused by the reflection of light from the jewel.
Iron jewelry was popular in Germany in the early 1800's during the war with Napoleon. The government replaced people's gold jewelry with iron replicas in order to further the war effort. Iridium and platinum are frequently alloyed together, since the iridium increases the workability of the platinum. Iridium is also used for the points of gold -nibbed pens. The change in the diamond is permanent.