Agates and carnelians are two varieties of chalcedony, which is a cryptocrystalline form of silica. The main difference between agates and carnelians lies in their colors. While agates can display multiple colors in bands, carnelians exhibit a rather uniform reddish-orange color. However, there can be some overlap in colors between certain agates and carnelians which leads some people to use the names interchangeably. Let's take a closer look at the similarities and differences between these two stones.
Agates and carnelians both belong to the quartz mineral family. Their chemical composition is silicon dioxide (SiO2). The banding we see in agates is caused by trace amounts of iron oxides and other mineral impurities that are present when the silica gel hardens into solid chalcedony. Carnelians get their reddish-orange hue from iron oxide impurities. So while agates can display iron oxides in their banding, carnelians have a more uniform distribution of iron oxide which gives them their distinct reddish color.
In general, agates exhibit a wider variety of banding patterns and colors compared to carnelians. Agates can form with clear, white, gray, yellow, pink, red, brown, orange, blue and green bands. The distinctive feature of carnelians is their solid reddish-orange color, though some carnelians may fade to a pale orange. When it comes to usage in jewelry, dark reddish-orange carnelians are usually preferred over the paler orange ones.
This is where there can be some overlap between carnelians and agates. Some agates may display reddish-orange bands that resemble the color of carnelians. These agates may get labeled and sold as carnelians in the gem trade. Conversely, carnelians with very faint banding may end up labeled as agates. So in some cases, reddish-orange agates and carnelians with minimal banding may appear very similar. The technical distinction rests mainly on whether the stone exhibits uniform color or noticeable banding.
In ancient times, red agate and carnelian were not always distinguished as separate types of chalcedony. The red stones were usually all referred to as sard after the source at Sardis. Later the term cornelian, derived from the French word for the cornel cherry, came into use for the reddish-orange variety. Over time, the practice developed of using the labels carnelian and sard interchangeably for the red stones. Nowadays, most sellers and collectors use the term carnelian for the reddish-orange chalcedony.
While casual buyers may not pay much attention to the technical differences, collectors and gemologists prefer to maintain the distinction between agates and carnelians based on the banding patterns. When shopping for red agate or carnelian, it helps to examine the stone closely to check for banding, especially if you have a preference for the uniform carnelian color versus a banded agate. Whether you call a particular specimen an agate or a carnelian, these durable red stones make beautiful jewelry and unique collector's items.
