The above photo is a picture of what a metamorphic rock looks like in a "thin section" slide under a microscope. Pyroxene and biotite mica are two of the minerals that are present in this rock. Geologists make thin sections of rocks to discover a rock's composition, texture, and mineral content.

Preparation
To look at a rock through a microscope, a "thin section" must be made from it. First, a sample of rock at least 3" per side must be taken from the field. The fresher (cleaner) the sample, the better. The sample must be cut into a block 1 3/4" long by 1" wide by 1/2" thick using a diamond bladed rock saw. The block is then epoxied to a standard glass microscope slide. The block is cut as thin as possible using a diamond saw. It is then ground down and polished until the remaining rock is 0.03 mm. The thin section can then be studied through a petrographic microscope.

Using a Petrographic Microscope
The petrographic microscope transmits polarized light through the thin section for study. There are two polarizing filters through which light passes. One filter is constantly in place. The other filter, the analyzer, is applied perpendicular to the first to study different aspects of the minerals. The spectacular colors of thin sections are seen with the analyzer applied. The colors are called birefringence. Magnification of the microscope varies from 10X to as much as 1,000X but typically, observations are made at 50-400X. The purpose of studying rocks through a microscope is to observe textural relations and to better identify the minerals.

The above "thin section" was made from a calc-silicate metamorphic rock taken from Hogancamp Mine in Harriman State Park.