Greg,
That's a hard question to answer specifically. There are many rocks in this world....
A general answer would be that they seem to be meta-pelites - meaning metamorphic rocks originating from fine grained sedimentary rocks. I say that because I have seen similar weathering patterns in meta-pelites. But confirmation could only be had from close examination of the grain structure. I spent three years in London University peering down polarized microscopes studying mineralogy, crystal formations, and lithology. Some of those lessons are burned into my brain....
Have you tested them with (a) some acid to see if they have any carbonate content (suggesting possibly a sedimentary origin), (b) a very strong rare earth magnet to see if there is any magnetite in them (indicative of a metamorphic derivation). Maybe you don't even have these rocks in hand.
If not metamorphic or sedimentary that leaves the possibility of an igneous origin. Maybe they are finely crystalized basalts? Again, one needs to have a close look-see.
Where do the photos originate from? Is there any context there?
If you had an ID and were able to date them in some way, you could get a geological map and find out where rocks of a similar age are located and then start the (long) search.
The best areas to collect veined rocks that look nice in aquascapes might be in outcrops of old (meaning eroded) mountain chains. The tectonic events (such as sea closing and subduction of tectonic plates) are well-recorded in the rocks of New England. Here we have anything from very pure marbles to volcanics from events 400-500 million years ago. The mica schists etc of New York State are beautiful to behold, often filled with garnets. The finer grained rocks are often veined with quartz. This type of area is better for finding suitable rocks for aquascaping than, say, a shield area.
Get a book of 'roadside' geology for your area and you might come up with some hints.
You're on the West Coast, right? As I remember, there are some interesting rocks up Highway 101 on the coast, and some nice ones inland in the higher ground. Some of the serpentines along the coast - and jade-bearing rocks - appeared good and if weathered rocks were selected from nearby streams, there might be some good candidates for aquariums.
Hope this helps.
Andrew Cribb