Directions: Read the following passage and answer the question below.
(1) What do diamonds, snowflakes, sugar, and salt all have in common? They are all crystals. Crystals are all around us. They are usually very, very small. We can often identify crystals by their regular shapes, their beauty, and their sparkle. Crystals come in a variety of different shapes. Some may be shaped like stars or flowers. Others may look like miniature boxes, tall spikes, or pointy pyramids. Some crystals sparkle, especially jewels like diamonds and rubies.
(2) Like everything else on Earth, crystals are made of tiny, tiny particles called atoms. These atoms combine to form molecules. Crystals can be made up of millions and millions of these molecules. Most molecules are far too small for us to see, even with a microscope. So why are the atoms and molecules in crystals so special? It’s because they always arrange themselves in neat and tidy rows. They do this over and over and over. If you could see the atoms and molecules in a crystal, they would look like repeating rows, set in neatly stacked piles. These orderly patterns are what give crystals their beautiful sparkle and special shapes.
(3) Scientifically, a crystal can be defined as a solid substance whose molecules are arranged in a regular, orderly, repeating pattern. These orderly patterns are what account for the many scientific and technological ways in which we use crystals today. This definition, though, doesn’t capture the breath-taking beauty, sparkle, and glow for which crystals are admired the world over. Though crystals of gemstone-quality, like diamonds and rubies, are rare, small crystals are a common form of matter. From the frost on a winter’s window pane to the tiny crystals of sugar we put in our morning coffee, crystals are all around us.
(4) Crystals occur in a variety of fascinating shapes, like spikes, stars, flowers, miniature boxes, pyramids, and more. The six-sided beauty of a snowflake and the smooth planes of an amethyst also reflect the inner order of their molecules. The tightly interlocked molecular pattern of carbon in diamonds makes the diamond the hardest natural substance in the world. Yet, a different molecular arrangement of carbon in the point of a pencil makes the carbon slide off easily as we write on paper.
(5) A crystal forms, or grows, by adding the same number of molecules to all its sides in identical patterns. This is the reason that the sides of crystals get larger but are able to keep the same shape. Some crystals form as molten rock cools and solidifies. Others grow as water evaporates around dissolved solids. Rocks are conglomerations of many small crystals. Each crystal is made of only one chemical substance, but a rock may contain crystals of many kinds. Molecules in volcanic rock or a water solution move around constantly at high rates of speed, bouncing off other molecules many millions of times each second. Under those conditions, it seems remarkable that any could ever line up in the orderly patterns needed to create crystals. Under the right conditions, however, crystals as large as the Hope Diamond do develop.
(6) In 1912, the German physicist Max von Laue discovered a way in which crystals could provide an even clearer window into the world of molecular structure. Von Laue discovered that passing an x-ray beam through a crystal’s orderly lattice of molecules would produce a pattern of spots (something like a shadow) that conveyed very precise information about the shape of the crystal’s molecules. Today, scientists use this method, called x-ray crystallography, to discover the shapes of complicated molecules like the proteins in human cells.
(7) Crystals possess many interesting properties. The way in which some crystals interact with light and electricity make them very valuable in electronic devices like computers. Other crystals start an electrical current flowing when they are squeezed or stretched. This phenomenon is called piezoelectricity. The quartz crystal vibrates at such a constant rate that it has become an important part in watches and other electronic equipment. In fact, until atomic clocks were developed, the quartz crystal clock was the standard for scientific time-keeping.
(8) Many crystals can be found in caves where underground rivers flow. In some caves, rainwater drips down from above and wears away the rocks. When the water evaporates, crystal formations are left behind. Often these crystals are in the form of stalactites (icicle-like formations hanging from the cave ceiling, made of many tiny mineral crystals) and stalagmites (cone-like formations rising from the cave floor). Molten lava also leaves behind deposits of crystals as it cools. Most gemstones, such as diamonds and rubies, are mined from caves and tunnels carved by ancient volcanoes. In 1994, the Cave of the Glowing Skulls was discovered in a rainforest in Honduras. Scientists believe these sparkling skulls date back to 1000 B.C. to a previously unknown civilization. The skulls are covered with calcium crystals which not only glitter but have protected the skulls from deterioration over all this time.
(9) Scientists are constantly looking for new ways to grow larger and more flawless crystals, since there are so many scientific and technological uses for crystals today. One successful, though very expensive, method is to grow crystals on board the Space Shuttle. Space-grown crystals are especially useful in computers because of their purity. The formation of large perfect crystals requires the precise and intricate alignment of vast numbers of molecules. Even the force of gravity greatly hampers this process. The lower gravity in space allows purer and more perfect crystals to be grown than could be formed on Earth.