Cobbles

If you have a smoothly rounded rock with no fusion crust, then it is not a meteorite. It is just a terrestrial cobble. Cobbles are common and do not have fusion crusts. Meteorites are rare and usually do have fusion crusts.

In geology, cobble or cobblestone is the word for any rock in the size range of 64-256 mm (2.5-10 inches). (If it’s smaller, it’s a pebble; if it’s larger, it’s 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.

All of these rocks are cobbles; the ones with “X’s” are typical rounded cobbles. Most of these are probably glacial cobbles, which tend to be more spherical. None of these rocks have fusion crusts. None of these rocks are meteorites.

These are photos of cobbles sent to me by people who thought they were meteorites. None has a fusion crust so there is no reason to suspect that they are meteorites.

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.

Stream cobbles, Arizona. These rocks have never seen a glacier. Stream and river cobbles are not as rounded as are glacial cobbles.

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 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, there is no way to recognize the rock as a meteorite just “by looking.” Put another way, none of the cobbles above look like meteorites so it makes no sense to ask if they are meteorites. Only expensive chemical or mineralogical tests could prove that it’s a meteorite.