Easy Achievement! Select N. Sanity Peak Level(2nd Level) on N. Sanity Island.
Start the level, Jump through some disappearing rock (Near Bonus Level) you'll come across a checkpoint where there is a clue for the gem. A drawing pattern on the rock below the torch.
After that, enter the next room and you'll see 3 x 3 platforms that disappear in a pattern, watch it closely and make a jump on the
Bottom Left Platform -->Bottom Middle -->Bottom Right -->Middle Right --> Top Right --> Top Middle --> Top Left --> Middle Left --> Finally Center Middle Platform.(Anti-Clockwise)
Red Gem will appear on the Top towards the main path, jump straight to reach Red Gem, and Collect it. Complete the rest of the level to unlock the Achievement.
Here, the Video guide for "Ruby Red" Achievement.