Diabase dike intruding pegmatitic granite in the Devonian of New Hampshire, USA.

Spectacular mineral collecting can be had at the Ruggles Pegmatite Mine near Grafton, New Hampshire, USA. The Ruggles Mine started off in the early 1800s as a muscovite mica mine, but it's now a tourist site. Its walls have beautiful exposures of a mid-Paleozoic granite pegmatite, having unbelievably large crystals. Well over 100 minerals have been reported from this pegmatite, but the most common rock-forming minerals here are quartz, potassium feldspar, biotite mica, muscovite mica, and schorl tourmaline. The Devonian-aged pegmatite at Ruggles Mine is one of several in the Grafton Pegmatite Field. These intrusions are part of the New Hampshire Plutonic Series, emplaced during the Acadian Orogeny.

The above photo shows a nice, vertical diabase dike intruding the pegmatite at Ruggles Mine. Unlike the coarsely-crystalline granitic host rock, the dike itself is composed of finely-crystalline, mafic rock. If this had erupted as lava from a volcano, it would be called basalt. As this material is intrusive, rather than extrusive, it’s not called basalt. Instead, this dark rock is called diabase, although it has the same general mineral content as basalt (= plagioclase feldspar and pyroxene).