Onyx is formed by the deposition of silica in gas cavities in lava, which results in the distinctive bands or stripes we see in the stone. They come in varied shades of red, orange, and brown, which often alternate with striking bands of white. The banding in onyx is straight, and is either brown or black and white, while curved bands occur in the variety of quartz known as agate. Onyx can be found in Uruguay and bordering regions of Brazil, Afghanistan, India, Madagascar, Peru and the US. Baja California, and northwest Mexico .