Gems and Geology

Gems catch everyone's eye as things of beauty and usefulness. Geology reveals part of their origin story in the minerals we stand on.

At Halibut Point geology means granite. Barbara Erkkila, our doyenne of granite, wrote about the industry (Hammers on Stone, 1980) and also fashioned granite into jewelry. Broadly speaking Barbara made granite into a gemstone. People who knew her personally think of her as a gem.

Red granite buckle by Barbara Erkkila

Halibut Point is Everyman's encounter with geology. The ocean, glaciers and quarrying have presented the bones of the continent in plain sight. We can't help wondering at least a little about it all.

Geologists wonder full time. At Halibut Point we've been fortunate to have  geologist John Ratti as Park Interpreter since 2005. John tries to make it comprehensible for the rest of us. As a subject area it is full of intricacies, mysteries, hunches and specialized vocabulary, just like all other branches of knowledge. It's not an exact science but it contributes to progress and the satisfaction of curiosity.

John Ratti pointing out glacial scouring and 'chatter marks'

Here's what it's like to have the geology bug: John says that every time he drives by roadwork or a blasting site, he wants to go back for a look. "If you're a geologist, that's like heaven." A free pass into the earth's crust. 

Andrews Point pegmatite seam

Adjacent to Halibut Point lies a celebrity feature in Massachusetts geology, the Andrews Point pegmatite that includes large 'blue' quartz crystals. These look like low-grade amethyst and may once have included gem-quality stones plucked away by earlier adventurers. By commercial standards what we see now lacks preciousness, falling short in terms of color, translucence and rarity. 

'Blue' quartz, close up

Interesting things happen in pegmatites that draw the attention of geologists all around the globe. Pegmatites seem to originate as a creative chemical soup at the interface of magma masses deep within the planet, perhaps in the presence of super-heated super-pressurized air and water trapped alongside. If these conditions remain for a very long time exotic crystals can form and grow.

 

John Ratti is always on the lookout for pegmatites. He split open a promising-looking boulder with a feldspar seam that he noticed on Halibut Point. Inside were two minerals that had caught his attention years before at the Harvard Mineralogical Museum, because of their Cape Ann associations. He has put them on display at the Visitors' Center.

Danalite and annite within feldspar, Halibut Point

Back in the 1860s Yale Professor James Dwight Dana described the very first examples of these minerals right here in Rockport. You can imagine that our quarrying industry provided an attractive hunting ground.  Danalite is named for him, the reddish crystal Fe²⁺₄Be₃(SiO₄)₃S above. Within the once-molten pegmatite traces of beryllium, silicon, and sulphur found just the right chemistry with iron and oxygen.

The black mineral in the photograph is annite, a member of the mica group. It contains large amounts of potassium. According to its Wikipedia article annite is of special interest to geologists in potassium-argon dating processes that determine the age of articles older than 1000 years. It also preserves an ancient record of the direction and intensity of the local magnetic field, which give geologists clues to rock origins that may have been thousands of miles away.

Park Interpreter John Ratti

Halibut Point is a walkabout storybook preserved by the Massachusetts Department of Conservation and Recreation. John Ratti enjoys taking us through the pages, even when he has to disappoint kids who came to catch frogs in the tide pools. He has the storyteller's gift of knowledge, excitement, and weaving together human history with the environment. John is the gem within the gem.