Definition of "rillet"

From the green rivage many a fall / Of diamond rillets musical, […]

1830 June, Alfred Tennyson, “Recollections of the Arabian Nights”, in Poems. […], volume I, London: Edward Moxon, […], published 1842, part V, page 25

First, a pearly white, unrelieved, then a trace of faint darkness here and there, and finally, the fine neatly printed equations in black, with an occasional red hairline that wavered through the darker forest like a staggering rillet.

1953, Isaac Asimov, Second Foundation (1971 Panther Books publication), part II: “Search by the Foundation”, chapter 8: ‘Seldon’s Plan’, page 86, ¶ 1