Rain-maker or rainmaker originates from native American tribal magicians (etymonline) and has come into English language around 1775.
It is used of a professional that is great in his field (seemingly magical) or somebody believed to be able to produce rain. Especially somebody generating a great deal.
See The Free Dictionary
"Inflammable" is derived from the verb inflame, which comes from in- and flame. The OED identifies the prefix as in-2, indicating the second definition of the prefix, rather than the third, which is the negation which is what you believed it to be from. I quote the right definition below:
used in combination with verbs or their derivatives, [...] with the senses ‘into, in, within; on, upon; towards, against’, sometimes expressing onward motion or continuance, [...] . (emphasis mine)
To inflame something is to set it on fire–i.e. to use motion to cause something to be in flames.
"Invaluable" does come from in- expressing negation, and thus it means not able to be valued. However, this can be interpreted two different ways—one, it is so worthless that it has no value, or two, it is so valuable that we can not put a value on it—like the concept of there being no finite number that is larger than the rest—you can always add one. The common meaning is #2, but the OED recognizes both definitions.
Neither of these examples are exceptions—the first is misleading because the two prefixes look identical, the second can be understood in two separate ways. The best way to figure these out—have a good dictionary at hand.
Best Answer
Yourn, ourn, hisn, hern, theirn all originate in the Anglo-Saxon Mid Anglian dialectal grammar (Cambs, Hunts, Beds) and contain the pronoun weak ending still present in mine and thine. My family originated in Cambs and evidently retained its dialect speech even when the Industrial Revolution obliged them to move to London in the 1830s. My grandfather said yourn, etc and my father said it (and I do!) whereas my grandmother's family and my mother's family came from different parts of Suffolk (East Anglian dialect) where they didn't say it.
They are not contractions of your one or your own at all. The Samuel Pegge quotation is correct - the ancient grammar lingered longer in Mid Anglia than elsewhere which is why it has survived. The Americans and Australians inherited it through colonization.