Goldstone is glass - with flecks of copper suspended in it. The glass is created in a reducing furnace (no oxygen), while copper salts are added. The furnace "smelts" the copper salts back to copper, which then crystallizes. The copper platelets suspended in the glass give Goldstone it's characteristic glitter.
Historically, Goldstone has been around since the European Renaissance. Stories say that it was discovered by early Venetian monks working at a glass factory (hence, the alternate name - Monkstone) - who were practicing alchemy. By accident, molten copper tipped into a container of molten glass. This crystallized into thousand of tiny crystals and formed a melt of glass with millions of tiny, golden light sparkles.
Thus, a happy accident gives us this sparkly stone today.