Rock Identification for Dummies

Igneous rock samples

After a week of overt techno-stupidity, it's time to move back into the wide world of the internet and see what dumbassery's out there that isn't published at Techwalla.com. As luck would have it, one of our researchers ran across what may well be one of the worst guides to rock identification ever printed, compliments of Kelly Townsend at, you guessed it, eHow. Here's our rebuttal of the post Kelly compiled in response to the search phrase "How to Identify Stones,"¹ which now lives (for unknown reasons) at GoneOutdoors.com.

We asked our staff geologist what the keys are to identifying "stones." He said that you classify rocks based on two criteria, texture and mineralogy. By texture, he says, he means the shape of the mineral grains that make up the rock and their relationship to each other. Once you have that nailed, we're told, you look at the mineral or minerals present in the rock. You can be either extremely general or outrageously nit-picky, but you can't be as dumb as Townsend, who said things like

Igneous stones don't have much layering or texture and are usually white, black or grey. Sedimentary stones are hardened layers and primarily brown. Metaphoric [sic] rocks are usually light in color and feature shiny layers. To identify stones, you must gather information about each stone, including its luster and hardness. The luster of a stone refers to its ability to reflect light. Luster is categorized as either metallic or non-metallic. If it is metallic, the surface will shine like metal. If it's non-metallic, it will have a dull surface. If a stone can be scratched with a fingernail, the hardness is approximately 2.5. If it can be scratched wtih [sic]a penny, it's 3.5. If it takes a knife blade, it's 4.5. Lastly, if glass can scratch the stone it's 5.5 on the hardness scale.

To which our geologist replied,

1. Color is not a diagnostic of rock type. 
2. Color is not diagnostic of rock type, and not all sedimentary rocks are layered or brown -- all the rocks in the picture above are igneous, even the brown ones.
3. Color is not diagnostic of rock type, and not all "metaphoric" rocks have "shiny layers."
4. No, Kelly, rocks don't have luster.
5. No, Kelly, rocks don't have a specific hardness. That's because they're (usually) composed of multiple minerals that all have different colors, hardness, and luster. Idiot.

So... Townsend confuses the identification of rocks with the identification of minerals. Not only that, but she seems to think that "location" is one of the characteristics you use to identify a stone, be it rock or mineral. Umm, no -- and for that sort of dumbassery, Kelly is the proud recipient of the Dumbass of the Day award.

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how_4488928_identify-stones.html

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SI - GEOLOGY