Tiêu chuẩn từ các kỳ thi thường đòi hỏi thí sinh có trình độ rất cao. Thí sinh phải luôn hoàn thiện mọi kỹ năng nghe, nói, đọc, viết. Tài liệu này sẽ giúp cho ta ôn luyện các kỹ năng đó thật tốt cho kỳ thi toefl. | Narrator Now listen to part of a talk in an archaeology class. Professor Most gems weren t formed by life processes so they re very durable. If you re one of the lucky few archaeologists who discover an ancient crown inlaid with say rubies you ll probably have to worry more about damage to the metal than to the stones themselves. But some gemstones are organic. They re more fragile and can present special problems if you ve dug them up and need to preserve them. One example is amber which formed millions of years ago from tree sap. The tree sap breaks down on exposure to air but if the tree died and was buried in an airtight space before decaying the sap could harden into amber. That s where amber gets its liquid clarity and smoothness. Uh now once it s hardened you don t need to worry about oxygen breaking it down. What you do need to worry about think of it as being like hardened wax. If it comes too near to heat it might melt or deform. Also contact with oils or strong acids can injure the surface and make it cloudy. The basic thing to remember is avoid sudden temperature changes and any contact with cleaning solutions and other such chemicals. Another organic gem is coral. Coral is sort of the skeleton of creatures from the ocean floor made of calcium carbonate often with carotene mixed in that s what makes it pinkish and orangish. You don t have to worry about melting coral but you do have to worry about scratching it. Calcium carbonate is naturally rather powdery so it chips easily. Also it s very porous so it absorbs liquids quickly. You need to make sure that you never soak coral in water or pour chemicals over it. Narrator Using points and examples from the talk explain how archaeologists must take the origins of amber and coral into consideration when caring for them. 310 CRACKING THE TOEFL SPEAKING PRACTICE DRILLS 311 Using points and examples from the talk explain how archaeologists must take the origins of amber and coral into consideration .
