How would you explain this sentence?
Among the blue rags turn up some blocks tinged with a stain of yellow or rust colour, which seem to be nearly as lasting as the blue; and every now and then balls of a friable substance, like rust of iron, called rust balls.
How would you explain “like rust of iron”? Does it mean the colour of the balls looks like the rust of iron? Or does it mean the friable substance is as friable as the rust of iron?
PS: this extract is taken from The Natural History of Selborne.