It could be a piece of fossil shell replaced by micro-crystalline quartz (chalcedony). But quartz (silicon dioxide) isn't as hard as many gemstones that are commonly cut with diamond. It has a Mohs scale hardness of 7, the same as popular semi-precious stones like agate or jasper (both forms of micro-crystalline quartz).
Note that the Mohs scale of hardness isn't a linear scale; the difference between a 7 (quartz) and 8 (topaz) is not as much as between 8 and 9 (corundum), which itself is much softer than diamond. Here's a chart that shows both Mohs scale hardness and true hardness: http://www.allaboutgemstones.com/moh...ess_scale.html