The Galaxy is the world's largest polished opal

Opal

Opal is an amorphous form of silica related to quartz, a mineraloid form, not a mineral. 3% to 21% of the total weight is water, but the content is usually between 6% to 10%. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most...

, certified by the Guinness Book of World Records in 1992. It was found at the Boi Morto Mine

Pedro II, Piauí

Pedro II, Piauí is a town and municipality in the state of Piauí in the Northeast region of Brazil.-References:...

 in north eastern Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 in 1976. It is today part of a private collection. The finished opal weighs approximately 3,749 carats

Carat (mass)

The carat is a unit of mass equal to 200 mg and is used for measuring gemstones and pearls.The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures, and soon afterward in many countries around the world...

, or 0.75 kg. It was carved from a grapefruit-sized piece of rough stone weighing 5,205 carats by Scott Cooley into a shape resembling a child's head. The weight of the finished stone was beyond the capacity of any gem measuring scale at the Gemological Institute of America

Gemological Institute of America

The Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...

and an approximation of its weight in carats had to be determined using a postal scale. The stone measures 14.0x10.2x4.1 cm (5-1/2 x 4 x 1-5/8 in).

Photograph of Galaxy Opal