A CS2 blue gem is not a separate skin. It is a Case Hardened item with a lucky pattern index. The Case Hardened texture mixes blue, purple and gold, and the pattern index decides where that texture sits. When blue covers most of the side you see while holding the weapon, the community calls the item a blue gem. The Counter-Strike wiki puts it simply: patterns with lots of blue are known as blue gems and are typically more expensive.
Case Hardened exists on the AK-47, the Five-SeveN, the MAC-10, every knife model and the Hydra Gloves, according to the item dataset. The AK-47 and the Karambit blue gems are the most famous. Only a small share of the 1,001 possible pattern indexes show mostly blue, which is why prices for those exact numbers are so high.
How are blue gems ranked?
Community tier lists group pattern indexes by how much blue shows on the playside, usually as numbered tiers or as a percentage of blue. The top tier means near-full blue and carries the biggest premium. Lower tiers still beat a normal pattern but by far less. There is no official ranking, so different lists can place the same pattern in different tiers.
The backside matters less. A pattern with a blue back and a gold front is not a blue gem, even if a screenshot taken from the back looks impressive. Ask for the pattern index and check the playside yourself.
Why are blue gems so expensive?
Three things stack. First, Case Hardened has to drop at all. Second, the pattern index has to fall on one of the few blue-heavy numbers. Third, collectors compete for a fixed supply that never grows for a discontinued case. The best-known AK-47 and Karambit blue gems change hands for far more than any normal copy, while most blue gems trade for much less.
Because of those sums, blue gem trading has more fraud than most parts of the market. Confirm the pattern index with a float checker on the inspect link, compare it to a known tier list, and use a marketplace with buyer protection rather than a direct trade with a stranger.