After my exciting discovery in Big Bend National Park (see post below) people have been asking: "how can you tell that this is a meteorite?" Which is indeed a very good question. As I stated before, my knowledge of geology is quite lacking, but there are some telltale signs with the stones I have in my hand. First, let me post a sharper version of the image. (The battery of the camera was on its last legs, so unfortunately, a lot of the pictures taken turned out to be blurry because the auto-focus could not keep up).
As you can see I have seven pieces in my hand. The first hint that these are not normal rocks is that they exhibit several characteristics of metal: they're heavier then stone and they're (somewhat) rusted. Iron, for example, rusts to an extent that even in the desert, a piece of metal will completely disintegrate in a couple of centuries. Iron turns into a crumbly rust that is washed away by rain or blown around by the wind. So, although the surface of the pieces have a rust color, they have not come close to disintegration. This means that the pieces of metal have not been lying around for very long, maybe a century, maybe more, but not for millenia.
Now, take a better look at a closeup of the second largest stone:
Can you see that on the top it has a different looking surface? You can see the front is smooth and curved, but the top is rough and dark. If this were due to weathering, we would expect to see this on more then one surface. But look how the edge from rough to smooth is sharply defined. What you are seeing on the top, is the area of the meteorite that was on the outside, i.e. it was burnt. As the meteorite hurls through the atmosphere, it gets incredibly hot. A lot of the meteorite simply "burns up" or vaporizes. In this case the meteorite was big enough to survive re-entry but then exploded. That is exactly what you see here: one side is burnt and charred, and sharply defined edges where the metal was ripped apart by the explosion.
Those sharp edges you can see clearly in both of the largest stones, and although I do not have a picture of this, the largest stone was also charred on the bottom, just like the image above.
Another interesting thing visible in the picture, is the rounded piece on the right. It looks different; no sharp edges or charring visible. Of course I do not know for sure, but it simply looks like a solidified piece of metal. It almost looks like a drop that was once liquid, and is now solid again. Given the tremendous heat and forces that come into play with a meteorite fall, molten metal is something that is quite likely to occur.
Lastly, if these pieces were from geological sources (i.e. not a meteorite) we would expect other minerals to be inside the metal; it would be contaminated. These pieces look pure and clean, they consist of only one type of material, not a mixture of rock and metal. Although not impossible to form naturally, it is unlikely.
So, in a nutshell, this is how I was able to determine in the field, without any further tests, that these are meteorites. Does this mean we are 100% sure these are actually meteorites?
Not yet at least. There is a test for metal meteorites. When a meteorite forms in space, they form in a free fall environment, just like astronauts in the Space Shuttle. Under those conditions metal forms a crystal structure called Widmanstatten pattern after the discoverer of these. This is what they look like:
For this you need to cut the meteorite, polish it, and then etch it with nitric acid. If the pattern shows, you have proven beyond a doubt that the stone is an actual meteorite.
Wonder if Self Chem has Nitric Acid available?