The rounded crystals are diopside, a member of the pyroxene family. They are surrounded by calcite. Notice the grid of lines in the upper crystal. These are the traces of cleavage planes viewed end-on; the nearly rectangular pattern they form is typical of pyroxenes and helps to identify them. You are looking at this crystal parallel to both cleavage planes; the same pattern does not appear in the two other diopside grains because they are oriented differently in the thin section. The different colors of the three diopside grains is also due to their difference in orientation.

Crossed polarizers