A big factor in the value of an agate, jaspers, and thundereggs is fashion.
Between 1950 and 1980 there was such an abundance of fine agate coming out of the ground in the US and Mexico that the supply seemed endless. People made a hobby of digging up agates or getting them at shows or at the hundreds of rock shops all over the country. Slabs of this material were used to create cabochons that were worth ten or twenty dollars which was a good profit from a top quality slab or a pound of Laguna rough, either of which might cost less than $5! A half nodule of beautiful agate had little value so good rough was usually cut into slabs. Now, cabs are almost worthless because of mass manufacturing, mostly in the Orient, and half nodules have become very expensive, so this represents a great change in taste and value. The large quantity of slabs left over from the earlier days are inexpensive in comparison to the value half nodules.
If you consider the agates from antiquity very few had flat surfaces. Modern lapidary equipment makes it possible to flat lap an agate so that the pattern in the stone is presented as if looking at a painting and that is now the style. In earlier times a domed surface was considered more desirable because the cutter could bring out the pattern and remove blemishes and thus create a piece that would be seen as the art of the lapidary. Sawing a rock in half and polishing the surface is a craft that requires great skill but little art. Agates that are cut according to the current taste have greater value, at least for the moment.
I cut the specimen below from a chip I picked up while I was working at the Morrisonite deposit. It had damage around the edge and the piece was thin. By doming it I was able to remove the damage and also save the pattern. The price would go up if it had the same pattern with a flat surface.
Another factor that affects price is the condition of the specimen itself. If the face of the agate has a crack, even a tiny one, or a chip on the edge or even a flake off the back the value of the specimen is less. Even minor flaws in the condition of the agate will affect the price. Other factors are the size of the piece and how it sits. A specimen that has to have an elaborate support is less desirable. A small specimen is difficult to appreciate but a large piece shows off the details of its pattern, size does matter. A specimen which has little contrast and is dark is rarely as valuable as an agate which has striking colors and pattern.
There are factors that depend on the life the agate has led such as having been in a famous collection or played a role in history. Some collectors desire specimens which have been featured in publications or major exhibits.
As everyone knows, from a dealer on the street to a graduate from the Harvard Business School, the main factor is supply and demand. If a material floods the market prices will drop and rarity will usually enhances value. Since agates are a surface phenomenon once a good deposit is located it is usually not long before it is worked out. Of course if social or political forces stop production or the source is very large and widely distributed it may be around for a long time. When a type of agate rough is no longer coming to market the price is sure to shoot up.
As with paintings aesthetics are very important. If viewers are struck by the art, the beauty and the elegance of an agate it can bring a fantastic price… the value of an agate is whatever buyers are willing to pay in order to add it to their collection and most collectors are very sensitive to beauty.
The agate below is very valuable because it is at the top of all the above criteria, you can go down the list and each criteria has been met in spades.
I look forward to hearing your comments on this topic, as a dealer I don’t set the prices, ultimately you decide.