Scottish National Dictionary (1700–)
BLACK STONE, n. (See quots.)
Sc. 1902 A. Lang Hist. of Scot. II. xxi.:
At that time  as in Glasgow [University] still, there were examinations upon the “black stone.” A seat with a stone in it still exists at Glasgow, a black capping stone at St Andrews [University]. Sc. 1927 Paraphrased from D. Murray Old College of Glasgow 81–92:
Used up to 1858 in Glasgow University in the expression “sit the Blackstone.” The black stone was a slab of black marble forming part of a wooden chair (still preserved in Glasgow University) on which a student sat for examination into his fitness for promotion into another class. The Black Stones of Aberdeen and Edinburgh have disappeared.
Hence blackstone chair, — examination (Gsw. 1909 J. Coutts Hist. Univ. Gsw. 343 and 344).[O.Sc. blak-stane, a dark-coloured stone used by students at their public examinations in some of the Scottish Universities. First quot. in D.O.S.T. 1581 and last 1659.]
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"Black stone n.". Dictionary of the Scots Language. 2004. Scottish Language Dictionaries Ltd. Accessed 21 Feb 2019 <http://www.dsl.ac.uk/entry/snd/black_stone>
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