A List of Rock Types and the Gold They Contain (Part 3)

In this post I'll continue with this series on rock types and gold they contain. Hopefully this information will clarify a few things for you and perhaps prevent you from overthinking things when you pick up "interesting" rocks out in the field or, as happened recently with some folks...in your driveway or around the house.


Obsidian is an extrusive igneous rock (lava) that forms as a natural glass and is usually black or greenish-black in color. In ancient times it was chipped and shaped to create projectile points, knives, and other weapons. Despite its parts-per-billion (ppb) gold content it has ZERO potential as a host rock for gold because it has not undergone any sort of chemical/structural alteration (metamorphosis) over time. From a gold prospecting standpoint you want to avoid recent lava structures or manifestations like obsidian because of this fact. Average gold content: 21 ppb.

 (Pegmatite with dark tourmaline crystals.)
Pegmatite is an intrusive igneous rock that's composed of interlocking crystals that are very coarse and large. Pegmatities and pegamatite dikes (i.e., veins) frequently contain or are associated with gem minerals like tourmaline, garnet, and beryl. Although pergmatites can be found in certain gold locations, they are not host carriers for gold per se. Average gold content: less than 1/10 ppb.

Periodite is a greenish-colored igneous rock that is coarse-grained and rich in magnesium and olivine with small amounts of iron. It has a very low silica content. In other words, it's nearly devoid of quartz or other crystalline elements. It is not a gold carrier or host. Average gold content: 4.6 ppb.


Picrite is a granular intrusive igneous rock that's also very silica-poor. Picrite is primarily composed of olivine and augite with small amounts of feldspar. It's not noted as a gold-bearing host rock in most instances. Average gold content: 20 ppb.

 ("Bull" quartz.)

(Mineralized quartz.)

(Free-milling gold in mineralized quartz.)

(Free-milling gold in "sugar" quartz.)

Quartz is the single most prolific mineral (notice I said mineral and not rock) out there and is found throughout the United States as well most other countries.It forms the crystalline structure of many other rock types and is composed primarily of silcon-oxygen tetrahedric crystals. It can be clear or translucent, solid white or white with minor coloring, or if heavily mineralized...multicolored with red, reddish-orange, orange, yellow, yellow-orange, blue, bluish grey, and on and on depending on the metals and other minerals it contains. Average gold content without additional mineralization: 12-100 ppb.

Rant Time

OK, listen up 'cause I'm going on a little rant here. Since quartz is one of the main gold hosts out there, this fact often causes confusion among less experienced small-scale gold miners and prospectors (or "civilians" with no mining knowledge whatsoever), including those who constantly hit me up about having quartz in their driveway or scattered around their property. If I sound peevish here it is for good reason, believe me. I just had a question sent me via e-mail three or four days ago along these lines....yet again. No matter how many times I write about this subject, how many pictures I show people, or how often I rant and rave about the fact that your rocks are most often just rocks and your quartz is usually as devoid of gold as your teenager's purse or wallet, I still get these queries. Once again, I refer those of you who just can't seem to grasp this simple concept to what I wrote in the first post of this series about the only SURE WAY of determining if your rocks (quartz included) contain gold or other precious metals/minerals.

Fuel to the Fire

Now I'm gonna add some fuel to this fire. What REALLY irks my ass is when I tell folks the truth about their rocks (based solely on what I can see in digital photos, mind you) and then they get indignant or suggest I'm wrong (always a possibility). If you don't think I know what I'm talking about in this regard WHY THE HELL ask me in the first place? Some of you have gone even further and have refused to believe what academics or geologists have told you...especially when they tell you the same thing I told you. Geeze Louise! There's just no winning this battle with some folks. If you're delusional that's OK. Be delusional. I'm not here to judge you or anyone else for that matter. But don't ask me to join the insanity, OK? I know this sounds tough and heartless but the truth of the matter is that I'm a very good person who gives an inordinate amount of my time and expertise to this blog and the people who e-mail me with questions. Essentially all for free, I might add. If you go to a doctor or lawyer and ask for their advice they're gonna charge you an arm and a leg and you'll pay them for the privilege. And if you don't like what they just told you, tough shit. You know, I always try to be courteous in my e-mail responses and let you down easy and I'll continue to do that. But I have my limits in this regard...that's all I'm saying here. Lastly, why not use your smart phone, lap top, or home computer to do a little research on your own this regard. You might be surprised by what you learn instead of taking the lazy person's approach and letting someone else always do the work for you. I'm here to answer your questions, yes. But repetitive questions that I've already answered and that you don't really want to hear the answers to? Hell no.

OK, now that I have that off my chest let's move on. Since quartz is such an important topic I'm going to devote the entire last post in this series to it. I'll talk about mineralization, what "sugar" quartz is, and why the old timers tagged certain types of quartz as "bull." Hopefully that will explain a great deal to those of you who aren't long-term small-scale miners or prospectors.

Best to all.

(c) Jim Rocha 2018

Questions? E-mail me at jr872vt90@yahoo.com


  1. Thanks for the nice rant Jim!
    Have a great day my friend.
    I appreciate that you have stayed on continue to write and share your knowledge!

    1. Yep...I sorta overdid myself there! No problem my friend. Hang in there and we'll keep going together.


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