Agate is probably one of the most argued about definitions in the rock community and I will break it down by what agate means to three types of people:
Rockhounds: Rockhounds consider an agate any cryptocrystalline quartz or chert chunk, seam, nodule or vein which is translucent. Different regions will consider different things agate depending on the pattern and where it is found no matter how it was formed.
People who sell rocks and Gemologists: An agate is classified by most people who sell agate as patterned crypto or microcrystalline quartz but just like rockhounds, it might not have patterns or be translucent.
Scientists: Agates to a scientist are one a subcategory of the quartz family and are formed by chalcedony or cryptocrystalline quartz. Agate is not really a term scientists use too much so much as descriptive names for how the agate formed like chert or chalcedonic quartz.
Its name comes from the Achetes River in Sicily, where agates were first found and noted in history. Usually banded in layers, or stripes, some varieties have "eye" markings, or specks of color, some have fossilized inclusions, and others are solid. Called the earth rainbow, the concentric bands of Agate form in nearly every color the earth can produce, including a colorless form.
Historically, agate has been discovered with the artifacts of Neolithic people, and was used as healing amulets and ornamentation dating back to Babylon and in the priests breast plate in the bible. Its medicinal uses continued through the ancient Greek and Egyptian civilizations, and spread throughout Africa and the Middle East into Russia. Agate sparked a world renowned stone cutting and polishing industry in Germany that flourished from the 15th to the 19th century, and exists today.
Agate is known for promoting inner stability, composure, and maturity.