an agate, used as a gem, having polygonal banding such that it suggests the plan of a bastion.
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How to use fortification agate in a sentence
Sometimes they are arranged in angles, presenting the fortification-agate.Scenes and Adventures in the Semi-Alpine Region of the Ozark Mountains of Missouri and Arkansas | Henry Rowe Schoolcraft
I found a piece of granite and a fragment of fortification agate in the sandy bed of the creek.Journal of an Overland Expedition in Australia | Ludwig Leichhardt
Some specimens have several distinct colours arranged in zigzag lines, somewhat resembling the fortification agate.Early Western Travels 1748-1846, Volume XIV | Edwin James