The GemHunter

Professor Hausel's Guide to Finding Gemstones, Diamonds, GoldRocks & Minerals 

Unusual Rocks

“I'm allergic to rocks hitting me in the face.”
Mike Rowe

Without rock, the earth would be boring. Imagine living on a gas giant like Jupiter - ugh! No rocks! Rocks have beauty, are formed from many individual crystals that represent solid combinations of atoms and some rocks include fossils and other replaced organic material. Rocks can even fill our pockets. As a teenager, I loved rock. Rocks are so useful that people even make their  homes from rocks: at one time, they may have been used in the transportation industry (although, this remains to be verified). And, if you have ever heard someone say, "you have rocks in your head", they are likely referring to our origin.

Rocks are naturally occurring aggregates of minerals, and classified as (1) igneous, (2) sedimentary, or (3) metamorphic. Igneous rocks crystallize from hot magma (i.e., basalt, rhyolite, granite, gabbro), sedimentary rock consists of pre-existing rock fragments (i.e., sandstone, conglomerate, breccia, shale) or are precipitated from solutions (i.e., limestone, dolomite), and metamorphic rocks recrystallize from pre-existing igneous, sedimentary and even other metamorphic rock due to tectonic forces (i.e., schist, gneiss, slate). Without rocks, we would not exist (yep, our heads and everything else came from rocks) and most everything we use came from rocks. Try to name one thing that did not come from rock. After you name it, think about it - what is the chemistry of the material you named, what kind of atoms does it contain. Remember those bumper stickers? 'If its not mined, its grown'.

When in college, I was lucky enough to work as an astronomy lecturer at a planetarium, and later study the first rocks brought back by Apollo missions. I can still remember some numbers used to identify the rocks - such at 14310-213. The rocks were mostly breccia, basalt and anorthosite and pretty much made from the same stuff terrestrial rocks are made from - same kinds of atoms, same types of minerals such as olivine, pyroxene, plagioclase, ilmenite, baddeleyite, geikielite, armalcolite; and dated between 3.16 to 4.44 billion years old. Scientists categorized them into suites such as: (1) ferroan anorthosite, (2) magnesian, (3) alkali, (4) granitic, and (5) breccia. Can't tell you how many basalts, breccias and anorthosite I looked at, but there were lots. And speaking of armalcolite - there were new minerals recognized on the moon, not found on earth at that time, and one was named in honor of the Apollo 11 astronauts: Neil Armstrong, Buzz Aldran, and Mike Collins. This, metallic magnesium-, iron-, titanium-oxide (Mg,Fe)Ti2O5 was one of three new minerals (along with tranquillityite and pyroxferroite) identified at Tranquiltiy Base. Years later, all three were found on earth in rare rock types. For example Armalcolite was later found in some Wyoming lamproites, some Montana lamproites and lamprophyres, some Western Australia lamproites and even in kimberlite in some diamond mines of South Africa, as well as elsewhere.

One unusual rock type, known as spinifex-textured komatiite is a very rare, but interesting rock. The took the photo to the left of a komatiite outcrop from the Hannas Lake Serpentinites in Western Australia. This is an ultramafic spinifex komatiite. If you find any rocks like this one, you may be near a gold, nickel or chromite deposit. These rocks are found in greenstone belts around the world, and are even present in the Seminoe Mountains and South Pass in Wyoming. 

Another uncommon rock is kimberlite. Kimberlite was named after the Kimberley Region of South Africa and has been found in Canada, as well as Colorado, Wyoming, Montana, Michigan, New York, Kansas, Kentucky and Tennessee. Then there are lamproites. The leucite hills near Rock Springs has several volcanic flows, breccias and necks of lamproite. Similar rocks are found in Kansas and Montana, as well as in Australia. And the more olivine-rich lamproites are the best lamproitic rocks for diamonds.