You are here:

Geology/Unknown object


Unknown object
Unknown object  
Mr. Reszka,

As a volunteer I help maintain our grade school baseball field in St. Louis, MO.  The field was created many years ago when a new road was built; the foundation of the field is basically everything that they had to dig up to make the road.  Every once in awhile something makes its way to the surface that catches my eye...

The attached item is 'hard as a rock' and I haven't seen anything quite like it.  At first look I thought it may be petrified chewing gum from some kid that spit it out on the field 25 years ago.  My next thought was that it may be a product of when the road was built.  Please let me know any insight you may have to this item.

Thank you for your time.

Hi Jim,
Looks like you have a rather nice piece of "chalcedony".  

Chalcedony is a cryptocrystalline silica mineral made mostly of quartz (SiO2).  It's usually translucent and looks waxy.  Like quartz, it comes in almost any color, but it's mostly found in white to gray to grayish-blue colors.  Browns, like your specimen, are not rare but they are less common.  

Many varieties of chalcedony are considered semi-precious and have specific names depending on the color; carnelian (red), chrysoprase (green), onyx (black and white bands), all forms of agate.  Brown chalcedony is called "sard" and is often used in jewelry.  

I thought, at first, your specimen might be amber by it's color and translucence.  But your description of it being 'hard as a rock' convinced me it couldn't be, since amber is relatively soft while chalcedony is much harder (can't scratch it with a knife).

Here are some images of chalcedony (mostly carnelian/sard);

Hope this helps.


All Answers

Answers by Expert:

Ask Experts


C. Robert Reszka, Jr.


I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.


I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

©2017 All rights reserved.