The titles of the illustrations are my own, but the details are from “Agates” by A C Bishop and W D I Rolfe, published by National Museum of Scotland and British Museum (Natural History), 1989.

At the time of publishing, Bishop was Deputy Director and Keeper of Mineralogy at B M (N H), and Rolfe was Keeper of Geology N M S.

1. “Fireworks”, a sagenitic agate from Dunbog in Fife. “Sagenitic” means containing sprays of fine tubules of chalcedony, a variety of quartz. (Fig.90, p 41)
   
2. “Tornado Approaching”, illustrating “tube of escape”, “rent” and “agate dyke”, technical terms in the formation of agates. (Fig.28, p 13)
   
3. “The Great Ogre”, a stalactitic and fortification Blue Hole agate. “Stalactitic” means that there were hanging pendants, during agate formation, looking like cave stalactites. “Fortification” means that there are sharp changes of angle in parallel lines, like the plan of a 17 to 18 C fortress. Blue Hole was an agate collecting site near the port of Montrose on the NE Scottish coast. (Fig. 54, p 25)
   
4. “Waves off the Island”, an agate from Ballindean. This reminds me of an aerial view of a triangular island experiencing a steady swell of waves. Ballindean is a coastal region in the Firth of Tay near the village of Inchture.  (Fig. 73, p 35)
   
5. “Volcanic Eruption”, is an agate from Binn Hill. Although it has been described as a “rising sun” agate, this seems too prosaic. Binn Hill is an inland site, near Glenfarg, about 8 miles, (12 Km) south of Perth. (Fig. 80, p 37)
   
6. “Storm Clouds”, a scenic agate from Heads of Ayr, a promontory on the west coast of Scotland, about 6 miles (10 Km) SW of the port of Ayr. “Scenic” agates resemble landscapes and seascapes (Fig. 105, p 48)
   
7. “The Ice Caves”, an agate from Norman’s Law, an inland site on a hill, 935 feet (285 m) high and about 6 miles (10 Km) NW of Cupar, Fife, in NE Scotland. (Fig. 81, p 38)
   
8. “The Bacon Cave”, a “thunder egg” agate from Oregon, USA. The name comes from a Native American folk tale about the origins of the egg-shaped stones. (Fig. 160, p 67)
   

Original image source: unknown - Animation by Optical Spy

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If you would like to see more great ambigrams check out our John Langdon Gallery.

Here we have three pictures of a bunch of pencils laying on a desk. At first glance they all look the same, but on second viewing you will notice that the lead in the large pencil on the left is different in each picture. So what's the illusion? Well, in one of these pictures the lead in the large pencil is the same size as the lead in all the smaller pencils. Can you tell in which one they are all the same? 

Animated by Optical Spy from an unknown source image

Wonderful illusion by A. Kitaoka

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If you keep you eye on one of the balls the other one seems to be all over the place. Then try it with the other ball.