Real agate, industrially coloured. Two centuries old as a practice, and rarely disclosed at retail.
Brazil, Uruguay, India, Mexico, Germany (Idar-Oberstein), Scotland
The dyeing of agate was industrialised at Idar-Oberstein in Germany in the nineteenth century and the methods have barely changed: soak the stone in a metal salt or organic dye solution, sometimes for weeks, then heat or treat it chemically to fix the colour in the porous layers. Blue comes from iron cyanides, green from chromium or nickel, black from sugar and sulphuric acid.
The result is genuine agate with added colour, and it is a legitimate product. The problem is only ever the description. A vivid blue agate slice sold as 'natural blue agate' is misdescribed; sold as dyed agate, it is exactly what it says.
Identifying it takes one look at the band structure. Dye penetrates the porous bands and not the dense ones, so a dyed slice typically alternates between intensely coloured layers and nearly white ones, with unnaturally high contrast. Natural agate's bands differ in tone but stay within the muted natural palette.
These apply to this model and not to the rest of Agate. Work them alongside the maker's general checks, not instead of them.
Dye enters the porous layers only, producing alternating saturated and pale bands with an unnatural jump between them. Natural banding varies more gently and stays in the browns, greys, creams and rust-reds.
Agate has no natural mechanism for electric blue, violet, magenta or neon green. If the colour is not white, grey, cream, brown, rust, black or soft blue-grey, it was added.
Cheap — a few dollars to low tens by size. Priced as decorative stone.
An orientation for honest, unrestored condition — never an appraisal, and never a quotation.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.