Trinidad in '30s, with beauty, power; Crown Jewel, by Ralph De Boissiere. London: Allison & Busby (distributed in America by Schocken Books, New York). 349 pp. $14.95.

This is a novel of extraordinary power and beauty. Set in the author's native Trinidad, its central theme is the growth of black political consciousness during the 1930s.

De Boissiere depicts the stark contrast between the poor workers and the wealthy colonial bosses, and compassionately describes the black people's courage as they struggle daily with humiliation and frustration.

Mass unemployment and subsistence wages make the time ripe for the black activist Le Maitre, an awesome leader who has committed his life to raising his fellows from their virtual slave status. Following the lead of British trade unions, he forms the ''Workers' Welfare,'' applying himself to the formidable task of promoting unity among a people whose identity has been systematically destroyed. He has to counteract the ''slave mentality'' of both blacks and whites in this ''jewel'' of the British Empire, where ''Britannia ruled. Ruled with chains that didn't clank, the slave mind fashioned to make men hate their own and turn from themselves in terrible disquiet.''

A dramatic and compelling story unfolds, and the author explores it with a cast of vivid characters.

De Boissiere captures tender moments of private intimacy and sickening instances of police brutality with the same artistic restraint. His acute ear for dialect, and eye for class and cultural characteristics, add to the authentic atmosphere.

His insights force us to examine our own prejudices and to confront our complacency toward those fighting for basic human rights in our own societies as well as abroad.