Sunday, October 4, 2015

A Serious Agate Find in Baltimore County, Maryland

Definitive banding and translucency are disinctive characteristics of  the only true Maryland agate find of which we are aware.  The locality appears to be around a contact point  between siliceous (quartz) rock and  serpentinite  rock within a small  plot between I-70 and  Catonsville where a Baltimore County geological map shows serpentinite to be dominant.

The metamorphosed quartz rock is rich in iron. Drusy quartz, often nodular, is frequently present, often on the surface of these rocks, and it fills vugs  inside them.  Banding is occasionally apparent, especially inside the particularly vuggy rocks.   Much of the material is similar to that collected half a century ago during the construction of I-70. The specimen pictured at right, which well-known Baltimore County collector Bob Eberle brought home from I-70  in the late 1960's, boasts  coatings of opal (var.) hyalite.

Shown at left is an example of the more recently discovered material before  being sawed or polished. When viewed under the scope, its banding appears as minute vugs filled with an iron rich quartz druse and other ferrous material as pictured in the   photomicrograph at right.

 A clearer macrocrystalline quartz druse accounts for  the translucency evident on  polished slabs as well as blue to
purple banding  in the images beneath our title post. This becomes evident when viewing the photomicrograph at left, which also suggests that the green color could bespeak sand grains at early stages of tectonic compression.

A potential for questioning the legitimacy of agate nomenclature asserts itself in various official definitions that specifically categorize agate as cryptocrystalline  with conchoidal fracture.  While a limited amount of chalcedony is present in a few of the Baltimore County specimens as shown at right,  most are primarily  quartzite with a healthy presence of macrocrystalline quartz.

 For clarification, we contacted John S.White, Past Curator-in- Charge of of the National Gem and Mineral Collection at the Smithsonian as well as one of the world's most frequently published authors on questions relating to the integrity of species nomenclature. His response:
 I have seen lots of agate boulders split without them exhibiting conchoidal fracture.  However, a chip from them might well do so.  In any case, if this is quartz and it exhibits colorful banding, then I see no problem with calling it agate.  Lots of agate has bands that are colorless quartz xls, especially on the inside of nodules as the very last thing to grow.  Quite often “agate” is in the eye of the beholder and lots of stuff that you nor I might not care to call agate is called agate by those who have it.  This material does appear to be banded and very colorful and I would not mind having a cab of it myself.

Clearly, the banding and the translucency define the material as agate. They also raise questions relating to Maryland's Official State Gemstone, known as "Patuxent River Stone" and described on the State of Maryland's Kid's Page as follows:
The Patuxent River Stone is actually an agate, a cryptocrystalline form of quartz. Found only in Maryland, the Patuxent River Stone's  colors of red and yellow reflect the Maryland State Flag. 
 Patuxent River Stone, of course, is neither banded nor cryptocrystalline. Even worse, until after the publication of our previously referenced post about Patuxent River Stone on October 29, 2010, it  was still being called"agatized dinosaur bone”  as well. In truth, Patuxent River Stone is nothing more than quartzite with some red and yellow hues.

No comments:

Post a Comment