Natural agate, which is a striped stone of a porous nature, takes a stain very easily. The white stripes are generally of a harder nature and not very porous, so after treatment the resulting stone may be one of a vivid blue, green, or red, with very marked white stripes which have not taken the stain. Agates are, of course, striped naturally, but the colors obtained are much deeper and more vivid than any that would be found in nature. The appearance of the stone is certainly made more attractive, and the dark blues, greens, and reds are almost permanent.
Onyx, a black and white striped type of quartz, is also treated to darken the black zones and so accentuate the differences in shades. The stone is first immersed in a solution of sugar or honey, which is kept at a constant temperature for several days, depending on the assumed porosity of the specimen. It is then taken out, washed, and soaked in warm sulphuric acid, the honey or sugar absorbed from the first soaking being converted into carbon. This gives the required black zones. It is fairly safe to assume that all agates and onyx showing marked differences of color within the one stone have been artificially treated; the natural stones are much less strongly colored.