How to replicate Transform.RotateAround error using quaternions?

Very simple question for those that know what the RotateAround error is.

I am coding some unity code into another program which only has matrices. I have found that the RotateAround was making very nice 3d spirographs by returning the maths to the origin, i don't know why...

I wish to copy the error in rotate around into a transformation matrix rotation, something like

"if angle > 180, angle = -180"

I have forgotten the behaviour of the transform.rotateAround function, i just know it makes a desireable error.