|Goldstone was created as a replacement for aventurine. Aventurine (the natural material) is a form of feldspar with shinny inclusions. The exact process is a trade secret discovered in the seventeen century.
The result is a glass (amorphous silica) with suspended copper crystals. It is likely that the copper begins in solution (maybe copper oxide) and is precipitated from the ceramic mixture under reducing conditions. The copper oxide being reduced to copper metal.
The color of the brown or "gold" variety is from the yellow-metallic look of the copper. The encasing glass is transparent.
The blue and green varieties are from the coloration of the glass itself.