THE EARTH'S ROCK FOUNDATIONS 55 



The most beautiful gem in the quartz group is opal. This is 

 a form of quartz found usually in volcanic rocks. It has a texture 

 that makes its luster exceptional, so that the stone gives off reflec- 

 tions of brilliant color that change according to the angle at which 

 it is viewed, now red, now green, blue, yellow. The most brilliant 

 opals are those that dart shades of red like flames, and such are 

 known as fire opals. 



Next to quartz the commonest rock-forming mineral at the 

 earth's surface is calcite. This is a carbonate of calcium (CaC0 3 ) . 

 It crystallizes in a variety of forms of which the rhombohedron 

 is the most common. It then easily cleaves along the planes of 

 the crystal faces in three directions, so that the pieces are bounded 

 by plane faces like a cube, but unlike a cube the angles at which 

 the faces meet are not right angles but are about 78 and 102. 

 The opposite faces are parallel to each other and alike, though 

 they are not squares, as in the cube, but quadrilaterals whose 

 sides meet at the same angles as the faces. These angles between 

 the faces are always the same in the fragments, so the fragments 

 are all rhombs. 



Calcite may be quite transparent, when it is known as 

 Iceland spar because such beautiful specimens of the mineral are 

 to be found in that locality. This spar has a peculiar effect on 

 light that passes through it, so that when a piece of the spar is 

 placed on an object, such as a printed page, each letter appears 

 double. The spar is said to be doubly refractive. 



Calcite, when pure, is transparent, translucent, or white, but 

 it may assume many different colors as it takes up various 

 impurities. It may be red or yellow from the presence of iron 

 oxide, or blue, green, and other tints from other substances. It 

 is a soft mineral with a hardness of 3, and so is easily scratched 

 with a knife. It decomposes readily in dilute acids, yielding an 

 abundance of carbon dioxide gas, so that when a drop of such 

 acid is placed on it, or a small fragment is put in acid, it effer- 

 vesces, the gas bubbles coming up through the acid as they do 

 in soda water. The softness, the rhombohedral cleavage, and 



