2026年4月托福机经套卷(第5套) - Read an Academic Passage / 2 Q3

The Mysteries of Opal Opal is a unique gemstone, known for its vibrant colors that shift as the stone moves. Unlike most gemstones, which form rigid, repeating crystalline structures, opals are amorphous, meaning they lack a fixed internal shape. This is due to the formation process, where silica-rich water seeps into cracks in rocks deep underground, leaving silica deposits when it evaporates. Tiny silica spheres create the internal structure of opals, and these spheres diffract light, separating it into colors, much like a prism does. The color and brilliance of opals depends on the size and spacing of the silica spheres. Larger spheres produce reds and oranges, while smaller spheres result in blues and greens. Because of Australia's unique geological conditions, it is home to over 90 percent of the world's opals. Opals are also fragile. Their high water content and noncrystalline structure make them prone to cracking when exposed to sudden temperature changes. Modern jewelers use techniques such as laser cutting and humidity-controlled storage to better protect opals. Ancient cultures believed opals had mystical properties and brought good luck. The Romans saw them as a symbol of hope and purity, while the Greeks thought they gave the gift of prophecy.