英语课文33.doc

Catalogue What is a Gem? Beauty Cut stone vocabulary Why are gems cut the way they are? Reflection and Refraction The Critical Angle Luster Fire Dispersion Durability Value Introductory ideas: Gems have been part of human history for over 20,000 years. Very early gems were generally of organic materials. Most gems used today are inorganic minerals. Early crystal gems were probably derived from alluvial sources. As found, gems are rather ordinary-looking, unlike cut gemstones. There are many different kinds of gems, and most come in many colors. Gems can be synthesized Gems can be enhanced and most commonly are . Stimulants are different from symthetics. Names: trade or commoreial names obscure the ture identify…… In this course we will consider what gems are, the factors that affect their value, where gems form, how gems are identified, why gems are colored, and other important gemology concepts such as simulants, synthetics, gemstone enhancement, and related issues. 1.What is Gem A gem is a naturally occurring material desirable for its beauty, valuable in its rarity, and sufficiently durable to give lasting pleasure. It should be naturally occurring, but it need not be crystalline. Beauty is determined by brilliance, iridescence, color, sparkle, and play of color. A gem should be durable against heat and common household chemicals. It should not be easily scratched or broken. Brittleness is a measure of the gem’s tendency to crack or cleave. How rare is rare? Typically, a diamond deposit yields about 5g gem/1000kg of mined material. That’s 5g per million gram! 2. Beauty Beauty of a gemstone is determined by brilliance, luster, fire and color. The first three quantities depend on the cut of the stone. Before we can understand why cut gems sparkle, we need to learn some basic terms to describe cut stones. Cut stone vocabulary Polished planau surfaces are referred to as facets. The midline of a faceted gem is called the girdle and may or may not be facetted. The are