In commerce, the word keystone, or keystone pricing means the retail price of an item is set at double the wholesale or production cost of that item. I have only really run into it when working in the jewelry industry. Does anyone know the origin of this terminology? Ideally I could find the earliest usage of it.
According to this post, it is stated to be from the jewelers' magazine, as @choster stated in the comments:
I've received an answer from a reference librarian, so I thought I'd pass it along:
The term keystone, meaning that 50% of the stated price should be considered to be the wholesale price, comes from the jewelry trade. It was originated in 1896 by Keystone magazine, a predecessor of Jewelers' Circular-Keystone, after subscribers had complained about the showing of dealer costs in a publication that customers might see on jewelers' counters.
The name of the magazine, Keystone, probably comes from the keystone cut, a fancy diamond cut whose outline is that of a keystone. (A keystone is the central, wedge-shaped stone at the top of an arch that locks the arch together.)
The magazine is still published, but is now simply known as JCK. As a sidenote, this business page states that the two magazines were separate until the 1930s (so as mentioned above, would have still been so at the time that the name passed into usage):
Jewelers' Circular-Keystone (JCK) is the jewelry industry's leading trade publication. JCK's origins date back 131 years to the original Jewelers' Circular magazine, which merged with the Jewelers' Keystone in the early thirties.