Agate rates a 6.5 to 7 on the hardness scale. The typical color of agate is a blue whitish grey. The most highly prized forms of agate are sard, cornelian, chrysoprase and jasper. Agate has many subspecies, which are distinguished by their colors and patterns. Carnelian is yellow-orange to orangey red and translucent. Chrysocolla is an intense light blue or blue-green. Chrysoprase is translucent, light to medium yellowish green, and was once considered to be a prized form of jade. Bloodstone is another form of agate, opaque and dark green with spots of dark red. Legends said bloodstone was formed out of drops of Christ's blood as he was crucified. Blue Lace Agate is translucent and is lavender to light blue with lacy white lines. Jasper can be different opaque colors, usually green, red, or yellow. Jasper can show various landscape-like patterns within the stone.

Unusual colors and markings of agate made it very highly regarded by the ancients and it was revered throughout history until the present. According to Pliny the Elder, agate was believed to make its wearers "agreeable and persuasive and give them god's favor." In the middle of the 1800's, amulets made from eye agate (brown or black agate with a white ring in the center) were so popular that agate cutters in Germany had no time to cut anything else. The eye was believed to take on the watchfulness of one's guardian spirit and to protect the wearer from the evil eye.