Goldstone

Goldstone is made, not mined. It was developed in the glassworks of Murano, Venice, in the late 16th or early 17th century; the term "pasta venturina" appears in a 1626 Venetian inventory, and the earliest known recipe was recorded by Giovanni Darduin in 1644. A base glass with about 4–6 % cuprous oxide (Cu₂O), plus lead and iron or tin compounds, is melted and then cooled over several days in a reducing atmosphere; the size of the copper crystals, set by this slow cooling, is what distinguishes goldstone from ordinary copper-red glass. In the 1860s French chemists published their own processes, including a green version with chromium oxide crystals.

Under 20x magnification the copper crystals are easy to see; their uniform shape and size (triangular and hexagonal platelets) and their even distribution through the glass separate goldstone from natural aventurine quartz and sunstone, whose spangles are mineral inclusions of irregular size. Gas bubbles are sometimes visible, as in other glass. In blue, purple and green goldstone the crystals may look silvery rather than coppery. Blue goldstone can resemble lapis lazuli.

Like most glass gems, goldstone is softer than household dust (hardness about 7), so wipe it with a lint-free cloth rather than scrubbing it dry. Mild soap and warm water are safe for untreated, unbacked pieces.

The Italian name avventurina is linked to "a ventura", "by chance", because the result was hard to predict: Giovanni Darduin, who recorded the first known recipe, called the process "unpredictable and deceptive". The German term Goldstein, from which "goldstone" derives, appears in 1634. The Miotti family of Murano became the main producers and guarded the recipe for generations; after the family stopped making glass, the formula was divulged in the early 19th century and production in Venice resumed. Popular stories that monks accidentally dropped copper into molten glass are, according to a Mindat article, apocryphal folklore not supported by evidence. Goldstone also gave mineralogy the word "aventurescence".