Opal at Bare Hills

Most likely opal is not uncommon as a coating on the serpentinite rock at Bare Hills along either side of Falls Road about a mile north of the Baltimore City line. Yet, I doubt that opal is on the radar of those prone to collecting here. At least, I've never heard anyone mention it, nor had I ever seen any Bare Hills opal. My only awareness of opal at Bare Hills was that Ostrander and Price's long out of print Minerals of Maryland (1940, Natural History Society of Maryland) named it as one of the species occurring at the two adjacent and long abandoned Bare Hills Serpentine Quarries as well as the surrounding barrens once dotted with chrome pits.

For the sake of reference, the image at left shows the westernmost and better recognized of those two former serpentine quarries along Falls Road. It was a good collecting spot until fenced off and declared off-limits by its owner, the Gerstung Inter-sports facility located at the end of Coppermine Terrace*. More accessible on the other (east) side of Falls Road, its presence mostly obscured by vegetation, is the second abandoned Bare Hills Serpentine Quarry. The ultramafic rocks comprising its talus, while not as interesting as the rocks I remember from years ago at the western opening, yielded my opal (var.) hyalite.

A dull blue-grey glint on a rock encrusted with tiny whitish globular dots caught my eye. Observing through the loupe, I assumed the dots to be magnesite or hydromagnesite, which ubiquitously forms massive coatings on similar rocks throughout the Bare Hills area. The globular presence was distinct enough that it prompted me to bring the piece home for a look under the scope. The right side of this post's title image shows the surface of this rock at 40x. No question that the coating is opal, whose obscurely reported presence at Bare Hills had mystified and fascinated me for more than 50 years.

Since the appearance of this rock was so similar to hundreds of rocks hereabouts, I suspect that opal may not be that elusive at Bare Hills. This certainly was not the first time I'd seen rocks in the area that were visually similar, at least at first glance. Otherwise, except for occasional birdseye chromite in weathered serpentine rocks along with a few veins of chrysotile, there seemed less that was likely to interest collectors than I recall from years ago at the quarry on the other side of Falls Road and the chrome pit dotted serpentine barrens surrounding it.

Though off-limits where not built over, my hunch is that those barrens on the west side of Falls Road, unlike those on the east side of Falls Road that surround the site of my recent visit, always were and still could be more interesting. They will be the topic of a Mineral Bliss post at some point in the future.

*It could be that the name of "Coppermine Terrace" was conceived in ignorance. In its entirety, Coppermine Terrace passes exclusively through serpentine barrens formerly worked for chromium only. The site of the former Bare Hills Copper Mine is approximately two miles away and set in a completely different landscape.