Instructions
Complete the levels "N. Sanity Beach", "Boulders", "Hog Wild", "Up the Creek", "Temple Ruins", "Whole Hog", "Sunset Vista", "Heavy Machinery", "The High Road" and "Fumbling in the Dark". These will each give you one White Gem without needing any other Gems of different colors to access hidden areas.
Complete the "Lost City" level to collect the Green Gem. Complete "Generator Room" for the Orange Gem. Complete "Toxic Waste" to get the Blue Gem.
Complete "Slippery Climb" to get the Pink Gem. Complete "Lights Out" to get the Purple Gem. Complete "The Lab" to get the Yellow Gem. Once you have all six of these gems you'll be able to go back to previous levels and use the gems automatically to get to areas that were previously hidden.
Complete the levels "Jungle Rollers", "Great Gate", "Upstream", "Rolling Stones", "Native Fortress", "Road to Nowhere", "Boulder Dash", "Cortex Power", "Lights Out", "Jaws of Darkness" and "Castle Machinery". Now that you have all 26 Gems you'll also be able to open the level "The Great Hall".