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.