a prickly plant with blue flower heads

The bright, steely blue of this three foot high plant's neat flower heads is particularly striking combined with the almost white stems. Each flower is an almost perfect globe-shape, and very prickly. The leaves are grey-green and covered in cobwebby hairs.

It is a prickly plant with blue flower heads.
It weighs a few ounces.

You identify a prickly plant with blue flower heads.
It is an unsellable plant.
It weighs four ounces, and it is worth about one busc.
Its keywords are plant, thistle, and thistles.