Crystal
Curved bands laid down in a cavity over an unimaginable stretch of time — named for a river in Sicily.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
An agate is a gas bubble in a lava flow that filled, slowly, with silica-bearing water. Each band is a pause in that process, and the concentric rings you see when the nodule is sawn across are a record of changing conditions over a very long period — one that geologists have never entirely finished arguing about. The fibres of chalcedony making up each band are microscopic and radially arranged, which is why agate is tough and takes a fine polish.
Theophrastus named it in the fourth century BCE after the river Achates in Sicily, where he says it was first found; the name has come down almost unaltered for two and a half thousand years, which is unusual for a stone. The town of Idar-Oberstein in the German Rhineland has been cutting agate since the fifteenth century, first from local deposits and then, once those were exhausted, from Brazilian rough, and it remains the centre of the European trade.
Practice treats agate as the steady, undramatic stone — stability, gradual work, holding a position. The banded varieties are further subdivided by pattern and colour into a great many named types, most of which are trade names invented to sell a particular deposit rather than distinctions with any basis in mineralogy.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Pliny the Elder, Natural History, Book 37 (c. 77 CE)
Pliny records that the agate was named from the river Achates in Sicily and was once held in high esteem, though by his own day it had become common and cheap. He lists a great many varieties by their markings — a habit the modern agate trade has never given up.
Carried forward
Common in contemporary practice
Used as a base or anchor in a grid rather than as the stone doing the work. The reasoning is that agate is slow and holds a position, and that something in an arrangement ought to be doing that.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.