Blue goldstone is a synthetic material recognized for its deep indigo base color and striking metallic glitter, an optical effect known as aventurescence. Despite its common use in jewelry and decorative objects, it is not a naturally occurring mineral but a form of man-made glass. The technique for creating goldstone originated with Venetian glassmakers, likely the Miotti family, in 17th-century Italy, where the process was closely guarded for centuries. It is produced through a precise and highly controlled glassmaking process.
Essential Raw Materials
The foundation of blue goldstone is a glass matrix, primarily formed from silica, commonly sourced as high-purity quartz sand. This sand serves as the bulk material that will be melted and cooled. The signature sparkle, present in all goldstone varieties, is created by adding copper oxide, or sometimes copper flakes or salts, to the raw material mixture.
The specific deep color of blue goldstone is achieved by incorporating coloring agents into the glass melt. Cobalt oxide is the primary additive used to produce the rich, midnight blue hue; manganese oxides can also be used for a purplish-blue tone. These metal oxides are mixed with the silica and copper agent in measured proportions before the entire batch is subjected to extreme heat.
The High-Heat Production Method
The manufacturing process begins by melting the raw materials together in a furnace at high temperatures, typically between 1,200°C and 1,400°C. This heat is necessary to fully dissolve the silica and metal oxides into a homogeneous molten glass. Once liquid, the mixture is stirred to ensure the copper and cobalt agents are evenly dispersed throughout the glass base.
A defining characteristic of this production method is the creation of a low-oxygen, or reducing, atmosphere within the furnace vat. This anaerobic environment is necessary because it chemically reduces the copper oxide within the molten glass. If oxygen were present, the copper would remain dissolved in its oxidized state, preventing crystallization in the next stage.
How the Sparkle and Color are Fixed
The blue color is fixed because the cobalt ions are chemically integrated into the glass matrix while in the molten state. Once dissolved, the cobalt oxide permanently stains the glass a deep indigo, forming the background for the metallic specks. This chemical bonding ensures the blue color is uniform and stable throughout the goldstone.
The sparkling effect results from a highly controlled cooling process known as annealing. Instead of being cooled quickly, the molten glass is held within a narrow temperature range for an extended period, allowing elemental copper to precipitate out of the glass solution. The metallic copper atoms, reduced in the low-oxygen environment, gather to form tiny, octahedral crystals.
This slow cooling is the most delicate phase, as the rate directly impacts the size and density of the copper crystals. A longer cooling curve yields larger, more distinct crystals, which produce a brighter glitter effect. Once crystallization is complete, the mass is cooled to a solid block, which is then broken and cut into pieces for beads and carvings.