By making the central character [of Birthstone] a split personality [D. M. Thomas] attaches his book to a long line of doppelgänger fictions, and thereby applies for a certificate of profound intent. Conrad's The Secret Sharer, Wilde's The Picture of Dorian Gray and Stevenson's Dr Jekyll and Mr Hyde are obvious ancestors, and behind them—stretching right across Europe—stands a host of others. But while Thomas implicitly refers to them, he seldom allows himself to make the most of their example. Where they are expansively fanciful he is inhibited by a fear of seeming absurd; where they are chillingly analytic he is rather ponderously explanatory; and where they successfully blend fantastic with realistic worlds he wanders uneasily between the two.
Saying so makes Birthstone's uncertainty sound a debilitating weakness. Given the mental state of its heroine and narrator, Jo, it is in fact the novel's greatest strength. By shifting between past and present, dream and fact, and opacity and clarity, the novel enacts her vain search for a firm sense of identity. The confusion is increased by Jo's tendency to be unaware of herself at crises in her life—and consequently to withhold information about the story upon which her existence depends. Even when relatively stable she is an unreliable witness, with the result that she persistently baffles and alienates the reader. Although her condition arouses sympathy, she is too perverse to elicit much affection.
It does not take Jo long to evoke this paradoxical response….
[Much] the same kind of ambivalence is provoked, finally, by the action which revolves round her. Thomas manages its deceitful twists and turns with a good deal of skill, and the end product is impressively unlikeable.
Andrew Motion, "Cornish Pastiche," in The Times Literary Supplement (© Times Newspapers Ltd. (London) 1980; reproduced from The Times Literary Supplement by permission), No. 4016, March 14, 1980, p. 296.