A CS2 Case Hardened is a finish that copies the look of heat-treated steel. The texture is a swirl of blue, purple, gold and silver, and the pattern index shifts where that texture lands on the weapon. Most copies show a mix of all the colours. A few land with blue covering the parts you see most, and the community calls those blue gems. Valve's Weapon Finishes guide lists Case Hardened as a patina style finish and explains that every paint gets random offsets and rotations when it is applied, which is why the pattern index changes the look.
Case Hardened appears on the AK-47, the Five-SeveN, the MAC-10, all 20 knife models and the Hydra Gloves, according to the item dataset. The AK-47 and the Karambit are the best-known because their blue gems fetch the highest prices.
Why does pattern index matter so much on Case Hardened?
For most skins, the pattern index makes a small difference. On Case Hardened it is the whole story. Two AK-47 | Case Hardened items with identical floats can differ by a hundred times in price if one is a plain mixed pattern and the other is a known blue gem. Traders track specific pattern indexes by number, and community tier lists rank them by how much blue shows on the playside. The Counter-Strike wiki notes that patterns with lots of blue are typically more expensive.
Gold-heavy patterns also have a following, though the premiums are smaller. A pattern with a lot of purple is usually seen as ordinary.
How do you check a Case Hardened pattern?
Use the inspect link with a float checker to read the pattern index, then compare it to a community tier list for that weapon. Do not rely on the listing photo alone. The playside, which is the side you see while holding the weapon, is what tier lists measure, and a photo of the other side can hide a plain pattern.
Since Case Hardened has a wide float range, wear tier still matters for normal patterns. On a top-tier blue gem, buyers often accept a worse float because the pattern is what they are paying for. Always confirm both numbers before agreeing a price.