Petrichor (n) is generally defined as the pleasant smell of dust after rain. Known to mineralogists as “argillaceous odour”, it has been noted to occur after the first rain following a long, dry period. The term is first coined by I.J. Bear and R.G. Thomas in the journal Nature to describe this smell, but it does not officially enter the Oxford English Dictionary until 1982. The compounds that produce petrichor are believed to be mainly comprised of oils and lipids created by vegetation during warm, dry periods that settle in the ground. Once rained upon, this compound is released into the air. The oils were discovered when extracted from samples of soil and sediments that produced high odours and were nicknamed ‘petrichor’ or ‘the essence of rock’. The two roots of this word are ‘petro’ and ‘ichor’.
Petro- , from the Ancient Greek, is a root denoting a noun that has some correlation with rock or is ‘of rock’. Petro, either πέτρος – stone or πέτρα – rock in ancient Greek are usually distinguished from each other by size. A stone is a size that can be thrown while a rock is boulder-sized. The Ancient Greek πέτρα was generally used as an equivalent to λίθος-stone from which comes –lith. From this come many words used in sciences such as monolith, laccolith or aerolith. Petri-, another form in Latin spawns petrifaction, petricolous and petrify, which all are very closely linked with rock. Ichor also goes back to Ancient Greek.
In Greek mythology ichor is defined as the liquid that is to Gods, as blood is to man. In 1676, Homer writes “From the wound out sprang the blood divine; Not such as men have in their veins, but ichor”. A famous instance of the word ichor is noted in the myth of Talos, who had a single vein of ichor plugged by a nail and protected Crete by throwing rocks at invaders. In 1900’s geology ichor is a fluid that exudes from magma, first referenced in 1926 by J. J. Sederholm where the granitization of rock is attributed to it. In the field of pathology ichor takes on a more unpleasant meaning as a fluid that is produced by open sores and wounds. However, the most consistent use of ichor is as blood, used 1638 and onward mostly linked to animals. The fluid was formerly known as the serum of blood but the definition falls more closely to blood in general.
Petrichor is among the most recent 3% of words added to the Oxford English Dictionary. It is interesting to note that it is specifically defined as a pleasant or pleasurable scent and its roots include ichor as the blood of the Gods.
 I.J. Bear, and R.G. Thomas, "Genesis of Petrichor," Nature, 30, no. 9 (1966): 869-879, http://journals2.scholarsportal.info.proxy.lib.uwaterloo.ca/tmp/11220438336385440813.pdf (accessed November 17, 2012).
 Oxford English Dictionary, "petrichor (n)." Last modified 2005. Accessed November 17, 2012. http://www.oed.com.proxy.lib.uwaterloo.ca/aboutthisentry/141889?nojs=true.
 Oxford English Dictionary, "ichor (n)." Last modified September 2012. Accessed November 21, 2012. http://www.oed.com/view/Entry/90831?redirectedFrom=ichor&.