Blue gem

The blue gem, also known as the Sapphire, is a rare type of gem belonging to the colored gems in the Crash Bandicoot series. Like the other colored gems, the blue gem is usually collected by completing a special task. Once it is recovered, the blue gem will unlock special routes by activating a platform or platforms in certain levels, known as gem paths.

Crash Bandicoot
In the original game, the blue gem is well rounded and the flattest of the game's gems.

The blue gem can be obtained in Toxic Waste by completing the level while smashing all of the crates and without dying after the first checkpoint. Once collected, it will unlock the gem paths on the levels Rolling Stones, Cortex Power, and Jaws of Darkness. Out of all the colored gems in this game, the blue gem is used the most for completion of other levels in the game.

Crash Bandicoot 2: Cortex Strikes Back
The blue gem appears with the same shape as the first game, though like all the colored gems it has become opaque instead of translucent.

It can be obtained in the very first level, Turtle Woods. Rather than breaking every crate, it is acquired by not breaking a single crate. If the clear gem has already been recovered, then if Crash breaks 'x' crates during the level, a box counter will appear at the end stating 'x/0' crates are broken, hinting towards the Blue Gem's true nature. It opens up the blue gem path in Hang Eight and part of the multicolor gem path in Spaced Out.

Crash Bandicoot 3: Warped
The blue gem appears exactly the same as in Crash Bandicoot 2: Cortex Strikes Back.

It can be obtained by completing the death route in Tomb Wader from the fourth warp room. It opens up the Blue Gem Path in Sphynxinator and part of the Multicolor Gem Path in Bug Lite.

Crash Team Racing
The blue gem appears similar in color to 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. Oddly and inexplicably, once it is collected, it is a substantially lighter shade of blue in the inventory screen.

The blue gem is obtained by completing the Blue Gem Cup tournament. If the player completes the CTR Challenges on the race tracks Sewer Speedway, Tiger Temple, Dragon Mines and N. Gin Labs, to collect all blue CTR tokens, then the Blue 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 blue gem. It is required to complete the game because the other four gems are needed to unlock Turbo Track. The blue gem also unlocks Komodo Joe as a playable character.

Crash Bandicoot: The Wrath Of Cortex
The blue gem has a slightly different model to Crash 2 and 3; it is less flat, features substantially more edges on the underside, and sports a lighter shade of blue, more akin to the color of the blue gem as it appears in the inventory screen of Crash Team Racing.

It can be obtained by completing the Death Route in Arctic Antics. It opens up the blue gem path in Tsunami.

Crash Bandicoot: The Huge Adventure
The blue gem returns here with a sprite identical to the clear gem, only a dark blue. It is hidden in Ruined. The death tornado spin is required to obtain it; Crash must jump from on top of the nitro switch crate back to the left reach it on a small hidden path in mid-air.

Crash Bandicoot 2: N-Tranced
The blue gem returns in this game, however for a different purpose then gem paths. In this game, there are blue gem shards, found in the levels Tiki Torture, Magma Mania, Mister Lava Lava, and Eruption Disruption. The blue gem shards used to unlock the level Fire Walker. In Fire Walker there is no crystal as it is replaced by a blue gem.

Crash Nitro Kart
Similar to Crash Team Racing, the blue gem is used as part of the Blue Gem Cup after completing the CNK Challenges where the player must earn points by racing on Barin Ruins, Clockwork Wumpa and Electron Avenue. Upon completion, Polar can be unlocked.

Trivia

 * In Crash 1-3, the blue gem is always located in a level whose name is two words long for which the first word starts with 'T' and the second word starts with 'W': Toxic Waste, Turtle Woods and Tomb Wader.