About Meteorites & Meteorite Collecting
Have you ever seen a "shooting star" streak across the night sky? Have you wondered if it may have landed somewhere? Well occasionally they do reach the Earth's surface and this is what is known as a "meteorite". Over the past several years meteorite collecting has become a much larger hobby and in some instances a profession. eBay serves as one of the larger market places for purchasing meteorite specimens and thousands of auctions run every week. Like Earth rocks, there are MANY different types of meteorites. If you are new to collecting or learning about meteorites, hopefully the information presented here by "Meteorites Australia" (meteorites.com.au) will go some way to helping you learn about these visitors from space!
Is the stone 'heavy for it's size'? Because meteorites usually have a large iron content they are quite heavy. Even meteorites with a relatively low iron content will still seem quite heavy compared to Earth or terrestrial rocks of the same size. Some meteorites are made completely of metal. These are called Iron meteorites, usually abbreviated to "Irons" and are very heavy for their size. They are primarily a mix of iron and nickel. Because of their high iron content, the vast majority of meteorites will also attract a magnet. Note that they themselves are not magnetic but rather a magnet will be drawn towards them.
Surface Features / Colour
Fusion Crust: When meteorites travel through Earth's atmosphere, they burn and form what is called a "Fusion Crust". The crust is usually black, grey to dark brown in colour. A weathered meteorite which has been on the ground for sometime will be more of a rusty/browny colour. Sometimes the fusion crust will have "Flow Lines" where the surface has melted and looks to have flowed around the meteorite. They look like fine wavy lines in the crust and will occasionally seem to originate from the front and stream towards the back. When meteorites hit the ground they will often chip parts of the crust off. Iron meteorites, when freshly fallen have a beautiful 'steely', blue/black fusion crust. Irons that have weathered will rust and form a rusty oxide layer. It may look like a lump of rusty metal and possibly flake at the surface. It could also be a mixture of these two things. Here is a Bensour (LL6 Ordinary Stone Chondrite) on the left and an iron meteorite known as Sikhote-Alin (IIB) on the right.
Rounded Corners: Meteorites are rarely ever a ball or sphere type shape, but actually quite irregular. One point to note is that they will usually have 'rounded' corners. Where the stones have large flat surfaces, it could be possible that it broke on entry and more specimens lay close by. Say within a kilometre or two! Notice the rounded corners in the fusion crust examples above.
No Holes: Meteorites DO NOT have holes in the surface. Many people believe stones that appear 'bubbly' are meteorites. They are most likely just pumice, scoria or something similar to it and often of volcanic origin. These types of stones are also usually quite light in weight. Only on extremely rare occasions have meteorite been found with holes and they are usually very small and very few in a particular specimen.
Regmaglypts or 'Thumb-prints': While meteorites don't have holes, sometimes they will have what are called "Regmaglypts" or more commonly 'thumb-prints'. If you could imagine the meteorite being a soft piece of dough or clay and you pressed your thumb into the surface, it would leave a shallow indentation. Generally speaking, if a meteorite has this feature, then the bigger it is, the bigger the 'thumb-prints' will be. Take note of the Sikhote-Alin iron meteorite below which shows this feature well along with fresh blue/black fusion crust.
Colour: When a meteorite is sliced or broken, the exposed interior surface is sometimes referred to as the "matrix". Meteorites vary extremely widely when it comes to the interior colour. They range from white to black, including red, orange, grey, tan/brown (common), silver (in Irons), yellowy or even a mixture of different colours.
Chondrules: Depending on the type of meteorite it could be, the interior could vary quite drastically. The most common is the Ordinary Chondrite which will generally show small round features called "Chondrules". (Pronounced: 'kon-drules') These can be different colours and sizes. The specimen below is NWA 2622 (LL4 Ordinary Chondrite) and is a great example of what chondrules look like.
Metal: As previously mentioned, most meteorites have a high iron content. In ordinary chondrites, there is nearly always small visible iron flakes. Again this feature can also vary in size and shape, but visible iron such as this is a good sign. On Earth, iron is extremely rare in its free natural form and is only found in a few places around the globe. The iron we see and use in our everyday lives, has to be manufactured and processed from iron ore. If the stone is completely iron inside, then it is a positive sign, however you would need to ensure that you don't have an old rusty man-made object. Many, many people confuse old metal objects for meteorites. Below is a slice of NWA 1287 (H6 Ordinary Chondrite) showing metal flecks throughout the interior.
Carbonaceous Chondrites: This is a very rare class in which some specific types would not display many of the above features. For example, the famous Murchison meteorite which fell in Central Victoria, Australia in 1969. This meteorite basically has a black interior with very small white to grey coloured chondrules. These meteorites are one of those rare exceptions to the rule! Below is a piece of Murchison (CM2 Carbonaceous Chondrite) showing small white chondrules and black matrix.
Achondrites: This name is given to meteorites which do not have chondrules and originate from different areas of the solar system. These meteorites vary quite widely and can have a plain, pale chalky interior to much more varied and colourful interiors with irregular inclusions or shapes. They can have fusion crust, metal and other meteorite traits but will not have the chondrules. Here is an example of two Achondrites below. On the left is the Eucrite, Agoult while on the right is the Howardite, NWA 1281.
Stony/Irons: These are RARE! If your specimen has any sort of 'crystal-type' pieces in the stone then it is probably not a meteorite. There is a rare class called Pallasites. Out of around the 30,000 different meteorites that have been found; less than 50 are Pallasites! If your stone has all of the iron meteorite characters AND contains crystals; it may be a Stony/Iron meteorite. The crystals are usually green/yellow to orange/black and can sometimes protrude through the surface. Below is the Pallasite, Imilac on the left and Mesosiderite, NWA 1827 on the right.
International Meteorite Collectors Association Inc. (I.M.C.A. Inc.)
The International Meteorite Collectors Association Inc. (I.M.C.A. Inc.) is an international non-profit association started for the primary purpose of authenticity. It's members abide by high standards set out in their "Code of Ethics" and "Bylaws". The I.M.C.A. can be visited at imca.cc while its members selling through eBay will usually display the their member number in an image.