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| | Agate |
 | | Much of the chalcedony in an agate is known, from the method of artificially staining the stone, to be readily permeable. |
 | | Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethystine, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity or geode. |
 | | When the deposits in an agate have been formed on a crop of crystals, or on a rugose base, the cross-section presents a zigzag pattern, rather like the plan of a fortress with salient and retiring angles, whence the stone is termed fortification agate. |
| www.jewelry-paideia.com /reference/ref-gemstone-agate-1.php (1967 words) |
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