Rockboard 60 boards have a dimension of 24" x 48" x 2" inches and are manufactured by Roxul. It is rigid mineral wool board similar to Owens Corning 705 at a density of 6 pounds per cubic foot.
Rockboard is rigid but not very strong, similar to styrofoam in strength.
Rockboard 60 is a great option for acoustic projects that require a rigid material.
If you need 6 or more pieces, order Roxul Rockboard 60 Case of 6 here.
For more detailed information, see Selecting the Right Acoustic Material for a comparison of these materials.