Blue Gem (Case Hardened)

Case Hardened is a finish (most famously on the AK-47 and Karambit) that mixes blue, gold, and purple patina in a pattern set by pattern index. Most pattern indexes produce a fairly even, gold-heavy mix, but a specific, well-documented set of indexes produce copies that are almost entirely blue — these are what the community calls "blue gems".

Because the blue-gem indexes are known, cataloged, and extremely limited, they're some of the most expensive individual pattern indexes in CS2 — a specific blue-gem AK-47 or Karambit pattern can sell for many times the price of an ordinary Case Hardened copy of the identical skin and exterior.

The mechanism is worth understanding, because it is what makes the premium permanent. When a Case Hardened item is generated, it is stamped with a pattern index from 0 to 999. That number decides where the patina texture is laid over the model, and it never changes: you cannot trade up, wear down, or otherwise move a copy toward blue. Two Factory New Case Hardened AK-47s sitting side by side at the same float can differ in price by four figures purely because of that one integer, which is why pattern is quoted alongside float on every serious listing.

Not all blue is counted the same way. On the AK-47 the community grades the top face of the receiver — the side you see in-game — so a copy with a solid blue top ranks far above one whose blue sits mostly on the magazine or the far side. Pattern 661 is the reference point, the so-called "Case Hardened Blue Gem" with near-complete top coverage, with 555, 179 and 387 among the other indexes that trade at a heavy premium. On the Karambit the convention is different again: collectors talk about the playside percentage, and 387 is the pattern that made the knife famous.

For buying and selling, three practical points follow. First, the exterior label tells you almost nothing here — a Well-Worn blue gem routinely outsells a Factory New ordinary copy. Second, the pattern index is not shown in the Steam client, so it has to come from an inspect link, which means any listing quoting a pattern without one is asking you to take it on trust. Third, the market is thin: for a given famous index there may be only a handful of copies in existence, so "the price" is whatever the last sale was, and comparing across marketplaces matters more here than on almost any other item in the game.

Blue gems are also not unique to Case Hardened as a concept. Any finish whose appearance is decided by pattern rather than float develops the same kind of sub-market — Fade percentage, Doppler phases, Marble Fade fire-and-ice arrangements, and the Crimson Web web placements all price on pattern in exactly this way. Case Hardened is simply the clearest and best-documented example.