Question: Why does properly processed chocolate have a uniform sheen and crisp bite?
About the article "Chocolate", section "Tempering".

Candidate passages:
The final process is called "tempering".
Uncontrolled crystallization of cocoa butter typically results in crystals of varying size, some or all large enough to be clearly seen with the naked eye.
This causes the surface of the chocolate to appear mottled and matte, and causes the chocolate to crumble rather than snap when broken.
The uniform sheen and crisp bite of properly processed chocolate are the result of consistently small cocoa butter crystals produced by the tempering process.
The fats in cocoa butter can crystallize in six different forms (polymorphous crystallization).
The primary purpose of tempering is to assure that only the best form is present.
The six different crystal forms have different properties.
As a solid piece of chocolate, the cocoa butter fat particles are in a crystalline rigid structure that gives the chocolate its solid appearance.
Once heated, the crystals of the polymorphic cocoa butter will be able to break apart from the rigid structure and allow the chocolate to obtain a more fluid consistency as the temperature increases.
This is known as the melting process.
When the heat is removed, the cocoa butter crystals will become rigid again and come closer together, allowing the chocolate to solidify.
This temperature in which the crystals obtain enough energy to break apart from their rigid conformation would depend on the milk fat content in the chocolate and the shape of the fat molecules as well as the form of the cocoa butter fat.
Chocolate with a higher fat content will melt at a lower temperature.
Making chocolate considered "good" is about forming as many type V crystals as possible.
This provides the best appearance and texture and creates the most stable crystals, so the texture and appearance will not degrade over time.
To accomplish this, the temperature is carefully manipulated during the crystallization.
Generally, the chocolate is first heated to to melt all six forms of crystals.
Next, the chocolate is cooled to about , which will allow crystal types IV and V to form.
At this temperature, the chocolate is agitated to create many small crystal "seeds" which will serve as nuclei to create small crystals in the chocolate.
The chocolate is then heated to about to eliminate any type IV crystals, leaving just type V. After this point, any excessive heating of the chocolate will destroy the temper and this process will have to be repeated.
However, there are other methods of chocolate tempering used.
The most common variant is introducing already tempered, solid "seed" chocolate.
The temper of chocolate can be measured with a chocolate temper meter to ensure accuracy and consistency.
A sample cup is filled with the chocolate and placed in the unit which then displays or prints the results.
Two classic ways of manually tempering chocolate are:
 Working the molten chocolate on a heat-absorbing surface, such as a stone slab, until thickening indicates the presence of sufficient crystal "seeds"; the chocolate is then gently warmed to working temperature.
 Stirring solid chocolate into molten chocolate to "inoculate" the liquid chocolate with crystals (this method uses the already formed crystals of the solid chocolate to "seed" the molten chocolate).
Chocolate tempering machines (or temperers) with computer controls can be used for producing consistently tempered chocolate, particularly for large volume applications.
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Couverture chocolate is a very high quality chocolate that contains extra cocoa butter (32-39%).
The higher percentage of cocoa butter, combined with proper tempering, gives the chocolate more sheen, firmer "snap" when broken, and a creamy mellow flavor.
The total "percentage" cited on many brands of chocolate is based on some combination of cocoa butter in relation to cocoa solids (cacao).
In order to be properly labeled as "couverture", the product must contain not less than 35% totaldry cocoa solids, including not less than 31% cocoa butter and not less than 2.5% of dry non-fat cocoa solids ; Couverture is used by professionals for dipping, coating, molding and garnishing.
The term "couverture chocolate" should not be confused with compound chocolate.
Products that contain compound chocolate have a lower percentage of solids and contain non-cocoa fats.
Some brands of couverture chocolate are packaged tempered, and others are packaged untempered.
Subsequent tempering may or may not be required, depending on the usage and the desired characteristics of the final product.
Couverture chocolate should not be substituted when semi-sweet, bittersweet, or unsweetened chocolate is called for in a recipe, as the increased cocoa butter content and the sugar content may alter the finished product. 
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This is part of a line of Nestlé products under the Crisp name, including Nestlé Crunch Crisp and Baby Ruth Crisp. 
 Crisp.
Nestlé also produces Baby Ruth Crisp bars, which are chocolate-covered wafer cookies, with a caramel-flavored cream and crushed peanuts.
This is part of a line of Nestlé products under the Crisp name, including Nestlé Crunch Crisp and Butterfinger Crisp.