Goldstone is not a natural gemstone, but is a man-made glass, also called Aventurine glass.
Goldstone comes in brown, blue and green.

Brown Goldstone is made by allowing copper salts to dissolve in molten glass, then cooling the liquid very slowly, allowing octahedral copper crystals to grow. It is these crystals that give the sparkle and colour.
For blue Goldstone/blue Aventurine glass, cobalt is used instead of copper.

The earliest known object made from a material that resembles aventurine glass was found in Iran and dates to the 12th century, although there are some who think it may have been used in Rome in the 1st century.
A Venetian glassmaker, however, would argue that the technique was developed in Murano in the 17th century.
Other tales say it was discovered by accident by an alchemist trying to turn copper into gold, or by an apprentice monk knocking copper shavings into a vat of molten glass...
.... pick your favourite tale and admire the ingenuity of man.