Cobbles

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If you have a smoothly rounded, hand-size rock with no fusion crust, then it is not a meteorite. It is just a cobble. Terrestrial cobbles are common and do not have fusion crusts. Meteorites are rare. Many, in fact, are rounded cobbles but most do have fusion crusts.

Nearly all of the photos below were sent to me by persons asking whether the rock was a meteorite. Note that none of the rocks has a fusion crust. We would not expect them to have fusion crusts if they were rounded by abrasion against other rocks on earth. In the absence of a fusion crust there is no way of telling just “by looking” if a rounded cobble is a meteorite unless it contains metal or has rusty spots. Testing would be required.

My backyard rock pile. All of these rocks are cobbles; the ones with “X’s” are typical rounded cobbles. Most of them are probably glacial cobbles, which tend to be more spherical. None of these rocks have fusion crusts. None of these rocks is a meteorite.

In geology, cobble or cobblestone is the word for a rock in the size range of 64-256 mm (2.5-10 inches). (If it is smaller, then it is a pebble; if it is larger, it is a boulder.) The word is commonly applied to any type of rounded rock (basalt, granite, gneiss, sandstone, etc.) that has been shaped into a spheroid (oblate or prolate) by abrasion against other rocks in a glacier, ocean, or river bed. Cobbles are common in mountain steams and as glacial till. River and glacial cobbles tend to be prolate; beach cobles are often oblate.

Black, gray, or red cobbles are often volcanic rocks like basalts. The shiny ones here are very fine grained and have been polished by abrasion with sand and other rocks. None of these rocks has a fusion crust.
Cobbles can form from vesicular rocks.
Cobbles can form from any kind of rock that is hard enough to survive abrasion against other rocks.
Cobbles can have stripes.
Cobbles on the beach of Lake Superior, Upper Michigan. These rocks are probably all glacial cobbles of a large variety of rock types that were formed farther north in Canada. Image credit: Randy Korotev
Stream cobbles, Arizona. These rocks, nearly all volcanic, have never seen a glacier. Stream and river cobbles are not as rounded as are glacial cobbles. Image credit: Randy Korotev

I am unaware of any stony meteorite that was found as a cobble or pebble and that had been rounded by abrasion against other stones on earth and, consequently, lost its fusion crust. There may be some from Northwest Africa. If anyone knows of one, let me know. I do know this. I once put a half dozen centimeter-size ordinary chondrites in a rock tumbler along with some terrestrial pebbles the same size, most of which contained quartz (=hard). After a week of tumbling, I could not find the chondrites when I opened the tumbler. Chondrites (=most stony meteorites) are not hard rocks. My little chondrites had all been pulverized by the terrestrial rocks. I would expect the same thing happens to any chondrite that falls in the ocean, rolls into a river, or is deposited in a glacial moraine. If the fusion crust is abraded away from an achondrite (Mars, Moon, Vesta) by terrestrial process, then there is no way to recognize the rock as a meteorite just “by looking.” Put another way, none of the cobbles above has a fusion crust so it makes no sense to ask if a rock that looks like a cobble is a meteorite. Only expensive chemical or mineralogical tests could prove that it is a meteorite.

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