River Rocks, Color, and Rings
Date: Spring 2012
I was walking a beach on the Deschutes River in Olympia Washington with a friend last night. We were picking up rocks and checking them out and we came across a few that were black with white, crystal-like rings around them. We also found a few that were other primary colors with secondary colored rings around them. What is the most likely cause of the white and black rings around the rocks? They are beautiful and I would like to know more about them.
It depends upon what you mean by rings.
Some of the river rock may be coated with deposits of lime from the waters or soils in which they rested for a while. The black color would be from minerals included in the lime crusts. These coatings should fizz slightly if you drop some vinegar on them and you would also be able to scratch them with a knife blade or pair of scissors.
If the rings are inside the rock and seem to be made of the same stuff as the rock, this could be from chemical attack on the rock. If it is white, they may have been bleached.
A final possibility is that the rocks formed inside bubbles of lava and are made of a variety of quartz. If this is true, then the rings represent different times in the creation of these rocks and show that the conditions they were formed under changed over time.
It is hard to decide what you have there without seeing them, but if you can get to a museum, Earth Science teacher or a local rock and gem or rock and mineral club meeting, they will be glad to help you out. Beware, though; they will try to convince you to become a real rock hound if they can. Of course, that will get you out on trips to find even more interesting rocks and may even lead you to cutting and polishing what you find! You could do worse.
Hope this helps.
R. W. "Mr. A." Avakian
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Update: June 2012