Apparently the Hemlock Stone was a stone hurled by the Devil at Lenton Priory, but fell 4 miles short. He was standing on the hill above Castleton in Derbyshire at the time. That's some throw even for the Devil. (see the 'At the Edge' webpage for more revolting detail).
According to 'Rambles round Nottingham' (v1, 1856), p224, the stone gets its name from the "tough green rag-stone or horneblend, called in the vernacular of the district 'hemlock stone'."
Also that (p225) "A sensible countryman, whom we encountered near the spot, when hard pressed to state the prevailing opinions regarding the Hemlock Stone, perseveringly declined to advance any but his own, which was, 'that it had been left by Noah's flood.' There can really be no other supposition."
It beats the usual 'Druidic sacrifice' explanations I guess.
In chapter 4 of JG Frazer's "Golden Bough", he mentions that:
On the Hemlock Stone, a natural pillar of sandstone standing on Stapleford Hill in Nottinghamshire, a fire used to be solemnly kindled every year on Beltane Eve. The custom seems to have survived down to the beginning of the nineteenth century; old people could remember and describe the ceremony long after it had fallen into desuetude.
The Hemlock Stone features in D H Lawrence's 'Sons and Lovers':
They came to the Hemlock Stone at dinner-time. Its field was crowded with folk from Nottingham and Ilkeston. They had expected a venerable and dignified monument. They found a little, gnarled, twisted stump of rock, something like a decayed mushroom, standing out pathetically on the side of a field. Leonard and Dick immediately proceeded to carve their initials, "L.W." and "R.P.", in the old red sandstone; but Paul desisted, because he had read in the newspaper satirical remarks about initial-carvers, who could find no other road to immortality. Then all the lads climbed to the top of the rock to look around.