A geode cut open and polished across the face, the outside left exactly as it came out of the ground.
The blue-grey and white here is the natural colour of Brazilian agate. Worth saying plainly, because Brazil is also the centre of the agate dyeing industry, and this particular blue-grey is the reason why. The stone is unusually porous, which lets dye move through it evenly, and the pale base takes colour without fighting it. Nearly every bright blue, pink or green agate slice on the market started as material that looked something like this.
Nothing has been added to this one. What you are looking at is what the silica was carrying when it arrived.
The contrast between the two surfaces is the point of cutting it this way. Polished face, raw rind, and the boundary between them showing how thin the shell actually is.
Mineral: Agate, banded chalcedony (silicon dioxide, SiO₂)
Crystal system: Trigonal
Origin: Rio Grande do Sul, Brazil
Hardness: 6.5 to 7 on the Mohs scale; durable, though the polished face will dull if knocked about
Treatment: None
Dimensions: 13.33 x 8.6 x 12.65 cm
Weight: 1.16 kg Note: Cut face polished, exterior left natural
You will receive the exact piece photographed.