Yellow gem

The Yellow Gem is a rare type of gem in the Crash Bandicoot series. While it is often called the Gold Gem, it's known to be a topaz. Along with the other coloured gems this gem is very powerful and opens up secret areas.

Crash Bandicoot
In the original Crash game, where the colored gems were earned on the same way as the clear gems, the Yellow Gem is obtained by breaking all the boxes in The Lab. It opens up the Yellow Gem Paths in Lights Out and The Great Gate.

Crash Bandicoot 2: Cortex Strikes Back
The Yellow Gem appears like a rather tall one. In this game it can be obtained in Plant Food by reaching the end of the stage before the timer reaches zero. It opens up the Yellow Gem Route in Sewer or Later and part of the Multicolour Gem Route in Spaced Out.

Crash Bandicoot 3: Warped
The sprite of the Blue Gem is exactly the same as in Crash 2. It can be obtained by completing secret route in Hang 'Em High, which opens when the player collected 10 relics. It opens up the Yellow Gem Route in Dino Might! and part of the Multicolour Gem Route in Bug Lite.

Crash Bandicoot: The Wrath Of Cortex
The Blue Gem gets a different sprite. The gem becomes more clear. It can be obtained by completing the Death Route in Gold Rush. It opens up the Blue Gem Route in Compactor Reactor.