The image above is of a photomicrograph (a magnified image of a thin-section) of an amygdule (amygdale) from a basalt of the 2.4 billion year old Thessalon Formation of Ontario. To prepare a thin-section a specimen of rock about 1 cm. across and a few mm thick is cut and cemented to a glass, microscope slide. The rock slice is then ground down to a thickness of about 0.03 mm. At that thickness most silicate minerals are more or less transparent.

Gas (usually steam) filled bubbles is commonly trapped as "vesicules" in cooling lava flows. When filled with minerals that crystallized from hot, aqueous fluids circulating through the cooling volcanic pile, the vesicule is known as an "amygdule".

The only basalt seen in the image is as very fine crystals (1) surrounding the amygdule. The outline of the original vesicle (gas bubble) is revealed by the line of white quartz crystals (2) that form an irregular oval enclosing the layers of bluish chlorite(3). Fibrous chlorite (4) fills the center of the amygdule.

The original deposition of minerals 2 to 4 in the vesicule probably took place within a few years to a few thousand years after the outpouring of basaltic magma. The final event was the deposition of epidote crystals (5) along a fracture that cut across the amygdule. This event probably occurred during the Penoken Orogeny ( a period of mountain building and metamorphism about 1.8 Ga (1.8 billions of years before the present.) The colours in the image are artifacts of the cross-polarized light produced by the petrographic microscope. Under plane-polarized and normal white light most minerals in the rock would appear colorless or pale-green to yellowish-green.