Oh! parent, parent! send no son to California except he be rock-solid adamant to all vice. Be her mountains pure gold, let them stand; send no kith or kin of thine to dig them, nor thyself turn towards them, unless armed like an old knight, cap-a-pie, for a desperate conflict, not with revolver and bowie knife, -lead and steel are of small worth, - but with manly principle, a moral coat of mail, light and easy, but impervious.This is the oldest reference I could find of rock-solid being used as a geoscience metaphor. It is an excerpt from the letter "Straws from the Gold Coast" written on June 26, 1849 in San Francisco and published in the "Daily Southern Cross". The article also offers an interesting account of what San Francisco was like not even one year after the official start of the Gold Rush:
A timeline from Google News Archive for this geo-pun looks like this:
just follow the link above and chose your favorite rock-solid reference.
In a more recent reference of this metaphor, Joshua Greenman describes his iPad experience with the words "Zippy web browsing, rock-solid feel: New Apple iPad is perfect down to the last micrometer".
One of most famous Romanian rock-solid geo-puns is "tare ca piatra, iute ca sageata", translated as "rock solid and fast as the arrow"; it is part of an ancient Romanian New Year's custom, when kids go around with a stick adorned with flowers and wish the loved ones all the best in the new year. One can find more info on this custom, which is known as "Sorcova", here.