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<p>[QUOTE=&quot;say_it_slowly, post: 3242538, member: 50&quot;]How about this?</p><p><br /></p><p><a href="http://www.stoneagetools.co.uk/what-is-flint.htm" target="_blank" class="externalLink ProxyLink" data-proxy-href="http://www.stoneagetools.co.uk/what-is-flint.htm" rel="nofollow">http://www.stoneagetools.co.uk/what-is-flint.htm</a></p><p><img src="http://www.stoneagetools.co.uk/images/urchin1.jpg" class="bbCodeImage wysiwygImage" alt="" unselectable="on" /></p><p><i>Internal flint cast of a sea urchin.</i></p><p><i><br /></i></p><p><i><b>Flint nodules in the chalk</b> changed over time as their outside acquired a whitish-coloured cortex or rind, possibly through ground water percolating through the chalk.It’s thought the cortex formed because the flint surface is slightly more soluble in some microscopically small areas than in adjacent areas, so there is differential dissolving at the surface which results in a microscopic sponge-like structure. The cortex can be 5mm or more in thickness. When people started making tools from flint, they often removed the external cortex to get at the fresh flint inside, although in some cases they left a smallamount of cortex to make some tools, such as scrapers and knives, easier to handle</i>[/QUOTE]</p><p><br /></p>

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