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.
Blue gems exist on the AK-47, the Five-SeveN and several knives, with the AK-47 and the Karambit the most famous. Only a handful of pattern indexes out of the full range count as top-tier, 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, often as Tier 1 through Tier 5 or as a percentage of blue. Tier 1 means near-full blue and carries the biggest premium. Lower tiers still beat a normal pattern but by far less.
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. Record sales for top AK-47 and Karambit blue gems have reached six figures in cash, though most blue gems trade for far 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.