How Do I Identify a Space Rock?

Originally published in ROM Magazine, Fall 2010.

I found a blackened rock that I think might be a meteorite. How can I tell for sure?

It is widely held that a picture is worth a thousand words. In the case of meteorites or more often meteor-wrongs—the all-too-terrestrial objects that are mistaken for meteorites—this is particularly true. Every year the ROM’s Earth Sciences section receives dozens of e-mails, letters, and phone calls from people who have found an unusual rock that they suspect is a meteorite. It’s easy to understand why. Written descriptions of meteorites usually specify a blackened, charred, or “burnt” exterior with surface pitting, sometimes referred to as thumbprinting. Less often “heavy” and magnetic” are also included as criteria.

The problem is that these “diagnostic” features are commonly mimicked by industrial by-products, such as slag.  Slag does have a quite striking, other-worldly appearance and is often found in unlikely places—which naturally leads people to suspect that it has fallen from the sky.  But given Toronto’s industrial past, and the number of landfills in which slag was a common component, it’s not all that surprising that slag turns up in seemingly impossible places.  To this day it is still reguularly found in railroad beds.

Like true meteorites, slag does have a melted or burnt appearance because it was at one time molten.  But unlike meteorites, which are charred from enduring the passage through out atmoshpere, slag is charred in a blast furnace, and it surface can be even more tortured than that of most meteorites.  Like meteorites, slag is often heavy and sometimes quite magnetic, since smelting is typically not 100 percent efficient, and iron may remain within it.

So how do we tell the difference?  The telltale characteristic is bubbles.  Gas bubbles are rare in true meteorites but nearly ubiquitous in slag.  Another difference is that slag is a glassy material that shatters with the characteristic scalloped or “conchoidal” fracture of glass while real stony meteorites break more like terrestrial rocks.

The ROM owes a debt to readers like yourself who are out paying attention to your surroundings and noticing odd, out-of-place objects.  This is how discoveries are made.  But it often takes a practiced eye to pick out the differences between true meteorites and Earth rocks that mimic their appearance.  Fortunately, the ROM offers a large display of “Meteor rights” in the Teck Suite of Galleries: Earth’s Treasures, where visitors can see many examples of the real thing.

Join us for the next Rock, Gem, Mineral, Fossil and Meteorite Identification Clinic on November 16, 2011.  Bring your space rocks and other treasures for a ROM expert to id.

Slag vs Meterorite: how to differentiate

Left: This sample of slag has large bubbles, or vesicles as they are sometimes called, which are very rare in real meteorites. Slag also fractures like glass. Right: Stony meteroites such as this one break not like glass, but like normal terrestrial rocks.