Actually, rock pictures is a new thing that is still in early research phases. The more I find out about them, the stronger inkling I have that a lot more is there. The current state-of the art commenced with Shaver's first successful infra-red photos. Those used wet canvas dusted with powdered glue for a negative.
The field is generating a new terminology. I simply call the first thing from a rock the negative to
avoid confusion. Otherwise a rock pic negative is another picture. The
following 7x8's are both mutually negatives for each other.
They are from common galena and projected onto a screen (pic of the simple rig I use for that enclosed- I have made a higher powered lamp house since then) and I hold a chunk of print paper on the screen for exposures from five seconds to two minutes, Since the first negative is a picture that saves having to reprint from a small negative and then print onto paper. Any 35mm SLR with bellows and close up lens (Spiratone has a very good one- it can be hopped up with a common Polaroid copying lens which fits like it was made for it works very well for rock photography. main problems are rock jiggle and lighting. The rock is usually closer than an inch to the lens and almost all have to be front lighted. A small bulb is most practical, small high intensity desk lamp) are practical. A smaller lens barrel solves most of that difficulty- any 8mm movie camera lens works fine for a close up lens. No adapters are available though so it needs to be Jury rigged with a cardboard disk or rubber grommet. Of course a great deal of magnification can done with the enlarger out to film grain and loss of contrast problems. There are literally thousands of kinds of rock- and so far I have just looked into a few. Any common high contrast fine grain conglomerate is easy to get fair to good pictures from without endless patience and specialized equipment. I've had the beat results from turritella ( which only needs magnification to 6X) and amethyst and galena.
The best images in amethyst ( and other kinds of quartz) are by shadow graphing- which requires a great depth of field- obtainable by extreme over correction of the lens, a very tedious process since the slightest change of the distance to the lens, angle to the lens or movement of the light source radically changes the pictorial content. Literally hundreds and thousands of different images can be gotten from the safe area on a rock. The type of light source' point, collated, negative aperture, or diffused also can bring out completely different images as can even lens stopping. Mirror surfaces on rock have the most picture content. Surface holographs can be scanned from them without a camera- using only a gadget similar to the sound track scanner in a motion picture projector. In [act literally any variable brings out even more images, even different spectrums- and rocks vary a lot- most local gravel tends towards the red end. There is so much to all .of this that there is a bit of a credibility problem. Especially since it is a vast mystery. There did all of those images come from- it is easy to estimate over a googol- what was (is) the mechanism that records them? Part of that can be resolved (as Shaver says logically) by the fact that many rocks resemble manufactured artifacts. Common silica is a good and durable recording medium, and would survive disasters. But that leaves what I call universal imagery - it is everywhere and on all scales of size. In rocks, emulsions, common glass and clouds, and a local photo shop owner mentioned he photographed some of it in ice. It is difficult at best to decide if an image was from a mechanical recording in the rock or not- the differences seem to be subtle. The best indicator is many items/scenes never appear in universal imagery (there is so much that it is impossible to see more than a very small portion though) and universal imagery lacks a lot of fine detail and shadings and often does not have a complete turn-around . Turn arounds are a difficult art form- but some rock pix index on rotation for as many as 32 images - commonly four. Another set of images can be found with the consolidating image effect- different viewing distances give different images- Salvador Dali's portrait of Voltaire is a handy example. It also works for size reduction of images on photographing- usually gives what I call "major" images, almost always a full head pic or a group scene or land/sea/space scape. Almost all are also 3-D images. It can be seen by viewing the pic at angles to nearly flat to the line of vision- it works best for me when holding a magnifying glass close to the pic at arm's length, as I am a bit far sighted. If the pic is from a common conglomerate at up to 10x on a 3x5 print, higher powered magnification ( a common projector lens ) gives excellent scenes at ground level for 3-D. In fact, quite often that works from ground level out through aerial views. Rock pix also have motion information- it can be seen by placing one on a phono turntable doing 16 rpm. Rock images can be projected directly with thin transparent chips set into 35mm slides. Once in a while I find one that responds to selective stopping for a small bit of motion picture. That is simply blocking off one side or the other of the lens with a small card, the edge of-a ruler etc. Best one so far shows an elderly man dressed in a rajah who has became poor getting up out of a chair- about equal to a dozen frames. There is bound to be sound recorded in rocks, but I haven't any idea how it was done or how to find it and transcribe it. Did find a bit of what looked like a standard Western Electric variable density sound track in a chip of obsidian- but it was small - it was beyond any available lens system to transcribe with any fidelity at all. It looked like a few phrases of speech, Then there is overlay and edge matching which makes another new image out of a rock image. Many prints have areas ( and that is often ruined by cropping) that can be matched to continue the image from one print to another. Or, ( where believability gets thin) another print of the same print turned upside down and Placed against the bottom of the right side up print makes a larger picture. I haven't tried selected sections that might match for larger pictures made up of identical sections that are triangularly or hexagonally shaped.
On seeing rock pix, there is more than one visual system. The one that is used the most is quick look. I suspect it depends a lot on a large mental stock of image models. Slow look is the quality system and it requires seconds to decades to operate. Average time for me-one to 30 minutes. I do perceive new things in photos I have gazed at many times through. Slow look is also .tied into dark adapted eyeballs. Some rook images visible only in dim bluish light are strangely demonic appearing. I also find that psychological good and unknowns influences what I see in the less obvious rock images. Artists tend to see more in rock pix. And besides all of that, rock pix have lots of ornamental virtues, especially since with table top photography, one can juggle colors a lot. Reflecting various colored light onto common clear broken quartz gives beautiful artistic images. Parts of Turritella resemble impressionistic art. Mexican crazy lace agate is always beautiful magnified 10x. It's rare, but common rock has miniature statuary and very common bias relief that is very easy to photograph.