According to Official Guinness Records,
The largest opal doublet measures 139 x 57.5 x 13.5 mm (5.47 x 2.26 x 0.53 inches) and weighs 798.39 carats. It is owned by Paul Christensen, of Christensen Fine Opals, Andamooka, Australia. OPAL DOUBLET
An opal doublet is a manufactured stone that is composed of two thin layers that are glued together. A thin layer of opal is glued on top of another mineral (usually a black onyx or ironstone, which enhances the stone's color), producing a stone that is less expensive than a solid opal. Doublets must be cleaned very carefully.
For a complete list of 2005 records, please visit 2005 Guinness Records in Australia.