Each skin in CS:GO has several parameters. If the float value is obvious – the degree of wear of the skin – depends on this characteristic, the pattern-index is a bit more complicated. Meanwhile, it is useful to know how exactly this parameter affects the appearance of the skin – because in some cases, its value can significantly depend on it.
Like float, the pattern index is determined randomly when creating an item. The source of the skin's origin has no effect on it. A game item may fall out of a briefcase or be given to a player based on the results of a match, but the pattern-index will be determined randomly either way. Like float, the pattern-index cannot be changed. It is not affected neither by stickers, nor by any operations with weapons, such as selling or exchanging.
It can be set from 1 to 999, and in relation to this parameter all skins in CS:GO can be divided into two large groups. A significant portion of weapon colorings have a pattern-index that doesn't affect anything. Whatever the number turns out to be, the texture will be superimposed on the weapon body in exactly the same way.
Another part of the skins has a pattern-index that directly affects the coloring. Let's take AK-47 Case Hardened as an example.
Depending on how this parameter is changed, the arrangement of colored spots on the case of the rifle – varies, and some variants may seem more pleasing to the players than others. The pattern-index determines which part of the texture will be used for a particular weapon.
For most skins, varying the pattern-index does not make a significant difference in price, as all its varieties will have about the same rarity. However, there are exceptions. For example, the Case Hardened series of knife skins, which turned out to be colored predominantly blue at certain values of the pattern index, became very famous. There is a known case when such Blue Gem Karambit was sold for more than 100 thousand dollars.
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Of course, this is not the only example of a skin that significantly increases in price with a rare pattern index. For example, in Crimson Web knives, the location of the web lines depends on its value. But from the looks of it, the options are similar, and it's not always obvious which one looks better and is more appreciated by players. To understand the cost of knives and firearms with rare pattern-indexes, it is often necessary to study the statistics of price changes on the trading platform Steam and compare several versions.