Conundrums of a Landscaper Series : Blackstar Gravel

I will preface this post with the statement that my wife thinks I am a total nerd. Landscape nerd. 

I was recently at my favorite rock yard, Whittlesey, buying a ton of Blackstar gravel (sometimes called Basalt) for the small commercial planting seen above. When I thought to myself, "where is Blackstar gravel from?"

The Unfortunate Question

I like to pride myself on the fact that I use native plants and source things as local as possible for my landscape business. I almost did not want to ask where it is from because Blackstar gravel is such a great product. The gravel is functionally one the best for paths and patios. Aesthetically, Blackstar offers a stark contrast to the green of grass or plants. In the right setting it can add a modern touch. If I knew the stone was coming from Arizona or some other far off state I would have a guilty conscious every time I sold it. As I thought about it more, all signs pointed to the fact that it must of come from a secluded beach on Hawaii or something. I have been the the farthest stretches of Texas and never seen a rock like this. The price is much higher compared to similar gravels. After these thoughts flooded my brain, I finally got the nerve to ask the rock yard manager.

The Origins of Blackstar Gravel

Uvalde County, Texas. Yes!

The yard manager let me know a bit of the back story, which I found interesting and thought you may too.

The manager said that the stone was actually Dolomite, not Basalt, like the other name it is frequently called (be sure to read to the end.) According to the manager, the stone is quarried close to Del Rio, Texas, but the reason the price is high is because the railroads use the same gravel to support the railroad ballast. The railroads use it because of its great strength and interlocking qualities. The railroads are driving the demand high for this product, not just the new fancy houses on the east side of Austin.

This new discovery about the Texas sourced Blackstar gravel, fascinated me for a few reasons.

1. Can it really be sourced in Texas? (Despite the good new, I still doubted the stone was from Texas.)

2. Why would a stone commonly be called Basalt when it is actually Dolomite?

First thing I did was call a Geologist buddy. When I mentioned that the stone may be Dolomite and not Basalt, it seemed to make sense to him based off his knowledge of Texas mineral deposits. There was one way to know for sure though.... Hydrochloric Acid of course.

He brought his family over the next day. As our children ran around and played, we ducked behind the garage with our rocks and diluted Hydrochloric acid. The idea was if we pour the acid on the Blackstar gravel and it fizzed it was Dolomite, if not, then it was something else. Dolomite is similar to limestone in composition, and both of carbonate minerals, which react to Hydrochloric acid.  

As both of our heads hunched over the stones with our ears close we poured the hydrochloric acid...... Nothing happened, I said, "those stones must have something wrong with them." We grabbed another handful, and poured some more acid on them, but this time we emptied the vile just to be sure. Nothing happened again. We walked back to our beer bottles as he told me, "I am not really the kind of geologist who is good at rock identification." I gave up on my research for the moment and enjoyed my beer.
 

Well, as any good researcher would do, I finally "googled" it. After some searching in obscure links, I found the clarifying document. As it goes, Blackstar is in fact Basalt. A very hard dark stone that forms from cooled magma. It is found in many places, but in Texas it can be found around the Chisos Mountains, Fort Davis, Guadalupe Peak, and down along the Rio Grande. The manager at the rock yard was misinformed.

Dolomite can also be found in Texas, but did not really match the description. It can be darker brown and grey, but not quite the black/grey of Blackstar. Dolomite is similar to Limestone in hardness and composition. Turns out when we did our Hydrochloric acid test we should have been able to know instantly that is was not Dolomite, which fizzes just like Limestone.

The Bottom Line

Blackstar is Basalt. Basalt is in Texas. Basalt is extremely durable, functional, and aesthetically pleasing in landscaping. See clarifying document below.

http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-1033031/txu-oclc-1033031.pdf

http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-1033031/txu-oclc-1033031.pdf