Yellow gem

The Yellow Gem, also known as The Topaz, is a rare type of gem belonging to the Colored Gems in the Crash Bandicoot series. Like the other Colored Gems, the Yellow Gem is usually collected by completing a special task. Once it is recovered, the Yellow Gem will unlock special routes by usually activating a platform or platforms in certain levels, known as Gem Paths.

This gem is often called the misnomer 'Gold Gem'.

Crash Bandicoot
In the original game, the Yellow Gem appears similar in shape to the Green Gem, with a flat and oblong shape, however, its ends are more flat than the pointed-end Green Gem, and thus it appears more rectangular.

The Yellow Gem is obtained by breaking all the crates and not losing a life after the first checkpoint in The Lab. It opens up the Yellow Gem Paths in Lights Out and The Great Gate. However, the Yellow Gem isn't exactly necessary to get to the Yellow Gem Path in The Great Gate, as there are two methods of bypassing the exit portal without the gem's use. Refer to Jumps Not Intended To Be Possible for further elaboration.

Crash Bandicoot 2: Cortex Strikes Back
The Yellow Gem appears drastically different to in the first game, similar in cut and shape to a Clear Gem, but taller and more robust. It is also much darker in color. Much like all the Colored Gems, it has become opaque instead of translucent.

It can be obtained in Plant Food by reaching the end of the level before the timer, which appears just before the first checkpoint and gives a time of ~1:03-1:13, reaches zero. It opens up the Yellow Gem Path in Sewer or Later and part of the Multicolor Gem Route in Spaced Out. The Yellow Gem platform is also the widest.

Crash Bandicoot 3: Warped
The Yellow Gem appears exactly the same as in Crash 2.

It can be obtained by completing the secret route in Hang'em High, which opens as level 27 when the player collects ten relics, although this is not obligatory, as the player can get from the main path to the secret route by two different methods. It opens up the Yellow Gem Path in Dino Might!, which in turn contains the secret warp for Eggipus Rex, and in addition part of the Multicolor Gem Path in Bug Lite. Again, its corresponding gem platform is much wider than the others.

Crash Team Racing
The Yellow Gem appears similar in color, perhaps slightly lighter, than in Crash 2 and 3. However, much like all five Colored Gems in the game, it has assumed the shape of a typical Clear Gem from Crash 2 or 3.

It is obtained by completing the Yellow Gem Cup tournament. If the player completes the CTR Challenges on the race tracks Dingo Canyon, Tiny Arena, Hot Air Skyway and Oxide Station to collect all Yellow CTR Tokens, then the Yellow Gem Cup will be available to play, where the four races can be raced again in a tournament. If the player wins the most points from all four races, they win the Yellow Gem. It is required to complete the game because the other four gems are needed to unlock Turbo Track. The Yellow Gem also unlocks Pinstripe Potoroo as a playable character.

Crash Bandicoot: The Wrath Of Cortex
The Yellow Gem has a slightly different model to Crash 2 and 3, it becomes more clear, smaller, a less intricate pentagonal cut and is slightly less distinguishable in shape to a Clear Gem, but nevertheless remains taller. It is also a much brighter yellow color.

It can be obtained by completing the Death Route in Gold Rush. It opens up the Yellow Gem Path in Compactor Reactor.

Crash Bandicoot: The Huge Adventure
Like all the game's Colored Gems, the Yellow Gem is identical in shape to the Clear Gems, but bright yellow in color.

It is located in Final Countdown. It unlocks the Yellow Gem Path in Ace of Space, which is the only section of the game that uses the space motorcycle.

Trivia

 * The Yellow Gem often appears later in the games, or generally more difficult to obtain. In the original game and The Huge Adventure, it is located in the final regular level of the game. In Wrath of Cortex, it is the final Colored Gem in level numerical order. In Crash 2, it is fourth Colored Gem of five in level numerical order, and arguably the hardest to obtain with considerable skill required. In Crash 3, it is the earliest Colored Gem in level numerical order, but possibly the last the player will obtain if the player saves time trials for after defeating Cortex the first time and obtaining the Crash Dash. Finally, in Crash Team Racing, the Yellow Gem Cup is composed of the 8th, 12th, 15th and 16th of the game's 16 main race tracks, some of the longest and most challenging tracks.
 * As it is often mistaken for the 'Gold Gem', it is perhaps highly appropriate that it should appear in the level Gold Rush in Wrath of Cortex.