Hard Crack Stage is 300 F to 310 F (150 C to 155 C)
Hard Crack: With a clean spoon, when the syrup dropped into ice water it separates into hard, brittle threads that break when bent.
You can actually hear a crack when the syrup hits the cold water. Once you take the syrup out of the cold water you can feel how brittle and sharp it is. Be careful as I poked myself in the video.