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| | Agate |
 | | In the formation of an ordinary agate, it is probable that waters containing silica in solution--derived, perhaps, from the decomposition of some of the silicates in the lava itself--percolated through the rock, and deposited a siliceous coating on the interior of the vapour-vesicles. |
 | | It sometimes happens that horizontal deposits, or strata usually opaline in character, are formed on the floor of a cavity after the walls have been lined with successive layers of chalcedony. |
 | | 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. |
| www.jewelry-paideia.com /reference/ref-gemstone-agate-1.php (1967 words) |
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