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Gilbert MainesPSA & SGC slabs are sonically sealed. If you use mechanical means to overcome this seal (such as prying, etc.) in addition to injury from flying shards (a distant relative of hanging chads) of plastic, you are at the risk of a sonic implosion; which under the right circumstances could become the focal point for a black hole - right in your hands! We all know the result of creating one of those.
Fortunately however, slabs open by themselves once you remove the sound which holds the halves together. Try wrapping the slab with sound absorbing material, and continue doing so until all of the sound is absorbed. Then just lift the halves apart.
Please note: everything above this line is BS.
Edited to add: PSA & SGC are easy. I pry and twist with a nail file.
BVG reportedly requires guts, but is simple. I have not tried it, but the prescribed method is to whack a screwdriver pointed at the label area with a hammer. I've hacksawed off the top and pried the halfs apart easily.
GAI I have not tackled.