Assertion: heavy bookend | /r/MadeOf | marble
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Source sentences:
text: Heavy bookends—made of wood, bronze, marble, and even large geodes—have been used for centuries; the simple sheetmetal bookend (originally patented in 1877 by William Stebbins Barnard) uses the weight of the books standing on its foot to clamp the bookend's tall brace against the last book's back; in libraries, simple metal brackets are often used to support the end of a row of books.