What is agate primarily composed of?

Agate is primarily composed of quartz, specifically a form of chalcedony, which is a microcrystalline variety of quartz. This composition is characterized by its layered or banded appearance, which is formed through the deposition of silica in a variety of colors and patterns. Agate's quartz content gives it strength and durability, making it a popular material for jewelry and decorative items.

The other options refer to different minerals or rock types: opal is a hydrated form of silica known for its play of color; calcite is a carbonate mineral often found in sedimentary rocks; and marble is a metamorphic rock composed mainly of calcite or dolomite. These materials have distinct properties and compositions that set them apart from agate, which is primarily recognized for its quartz component.