Llanite

The brown, fine-grained groundmass consists of very small quartz, feldspar, and biotite mica crystals.

Llanite comes from a hypabyssal porphyritic rhyolite dike that intrudes Precambrian metamorphics in the Llano Uplift of central Texas.

The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions.

[1] The geology of northeast Africa is very similar to that of Texas, and many of the two regions' minerals and fossils are only found in these two locations.

[2] A dike of llanite crops out on Texas State Highway 16 about nine miles north of the town of Llano.