Butterfly Knife Stained Minimal Wear is a Covert knife skin in CS:GO that was added to the game in July 2014 as a part of the Operation Breakout Update. This particular skin takes inspiration from the real-life butterfly knife, and as such it looks very similar to the real-life version of it, featuring a folding blade with a single cutting edge.
In terms of design, the Butterfly Knife Stained skin features a chromed blade and a dark metal handle that have gotten their patina by purposely exposing the blade to mustard and lemon juice. The Butterfly Knife Stained can be obtained by opening the Operation Breakout Weapon Case. The knife comes in all five exteriors and can have StatTrak attached to it.
In terms of value, it usually goes for upwards of $400 depending on the exterior as well as the attachments. Trade your old skins for those you've dreamed of for a long time!