This question is extracted from a real TOEFL Speaking Test of 2016. It has a sample answer, which can be used as an answer template for this type of question.
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AUDIO TRANSCRIPT OF THE LECTURE IN THIS TEST
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. Now, once it’s hardened, you don’t need to worry about oxygen breaking it down. What you do need to worry about is...well, 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.
