The logo symbolizing the Chaldea Security Organization in Fate/Grand Order.
There are actually two distinct logos associated with the organization:
A C shape embellished with a laurel along the left curve (post #2962495)
Two diamonds interlocked with each other vertically (post #4757212). This may also be seen as a diamond-shaped G over another diamond meant to symbolize an O, for Grand Order (post #5017643).