Pearls Course: Lesson 4. The Structure of Pearls

Transcription

1 Pearls Course: Lesson 4 The Structure of Pearls

2

3 Introduction There are two main substances that oysters secret to make a pearl. The first is called nacre, the second conchiolin. By alternately secreting these two substances, any mollusk can make the mother-of-pearl that makes up their shell. This is also why pearls can be found in so many different species of mollusk, and in so many diverse places in the world. The key point about these substances is that they are paramount in determining the quality of the final pearl product. So before we can start grading our bucket of newly harvested pearls, we need to understand what we are grading on a much finer scale.

4 Nacre Nacre is the term used to describe the total coating of the pearl, but is actually two parts. This is a combination of two substances the oyster lays down on the pearl. The first part is calcium carbonate that the oyster forms into a mineral called aragonite. This has the same chemical makeup as a roll of TUMS antacids. In fact, one of the medicinal uses of pearls hundreds of years ago was to crush pearls into a glass of wine and use it to treat gastric distress for the king or queen. Of course, since these were natural pearls, this was perhaps the most expensive glass of antacid that has ever been taken for heartburn. But it did work. The calcium carbonate in the aragonite of pearls is arranged in prismatic crystals that form in layers. The oyster will secrete a layer of nacre to cover the irritant, then lay down a layer of conchiolin.

5 Conchiolin Conchiolin is an organic protein that the oyster secrets on top of the nacre that effectively serves as an adhesive agent for the nacre layers. It is the orderly arrangement of these layers, the total thickness of the layers, and the smoothness of these layers that determine the quality of the finished pearl.

6 Below is an image to demonstrate how the aragonite/conchiolin are laid down on the surface of the starter seed. The light blue area is the starter seed that we put into the oyster. The rectangles are the aragonite crystals that form as prismatic crystals. The blue lines are the layers of conchiolin bonding protein that the oyster laid down to bond the layers together.

7 The important part of this is that the aragonite is arranged in prismatic crystals. And what does a prism do?...it breaks the light into the spectrum of colors, which is exactly what the aragonite does with pearls. This breaking up of the light on the pearl surface is called orient...or luster. Orient You may hear the term "luster" used in current pearl circles. But the traditional term for the light refraction on the surface of a pearl is called "orient". Therefore, this is the term we will use because the orient of the pearl is the main reason to own pearls, and the main ingredient of beauty in a pearl. There are two important aspects that go into creating high quality orient.

8 The first is nacre thickness. Pearls with very thick nacre layers have almost a metallic appearance, with a highly reflective surface on which you can almost see mirror images of things in the room. This is the mark of a pearl with thick nacre layering. The second is the smoothness of the nacre layering. Nacre that has been laid down in lumps or uneven layering will show distorted images and not give the high quality diffraction that is required to create really nice orient. These two factors will be covered more in the grading section. For now it is important to know that the uniform depth of the nacre layer creates a high quality orient in a pearl. And this orient is the result of the diffraction of light off the surface of the pearl.

9 Below is an image of two pearls that demonstrate the importance of orient. Below right the reflections of the lights around the pearls is sharper than the one below left. This is the indicator for a pearl with thicker nacre and therefore a greater degree of luster or orient.

10 Structure The end result of the layering of aragonite and conchiolin by the oyster is very different in a natural pearl compared to a cultured pearl. In the natural pearl, the entire pearl is layer upon layer upon layer of nacre that makes a solid pearl composed completely of nacre. At left is an image to demonstrate the crosssection appearance of natural pearls.

11 Cultured pearls, however, are much different, mainly because the vast majority of the pearl is the starter seed and not nacre. In fact, in very fine quality pearls of 7mm, for instance, 6.5mm is starter bead and only.5mm is nacre thickness. Consider just how different these two pearls are. One is solid nacre that can take decades to make, the other is simply an outer layering of nacre that takes a maximum of 3 years...usually much less. Compare the structure of the cultured pearl below with the natural pearl above. Regardless of the type of pearl you are inspecting, the above process has occurred in some form. The process of culturing pearls applies to all types of cultured pearls...although the processes will vary with some. Specifically non-nucleated fresh water pearls, but we will get to those in a while. For now, it is important for you to know the chemical makeup of nacre, the structure of the nacre, the orient created by the nacre, and the difference in the structures of cultured and natural pearls.

12 Now that we understand what makes a pearl structurally, and how that structure provides the foundation for the beauty of the pearl, we are ready to learn how to grade our bucket of freshly harvested pearls. But first, let's take a practice exam on what we have covered so far. Let s test your knowledge with a Quick Quiz on the next page. Please note: There are four possible answers to every question. You may have to scroll down using the control on the right to view all possible answers International School of Gemology. ALL RIGHTS RESERVED. This information is intended for the sole use of registered students of the International School of Gemology. It cannot be duplicated or distributed to anyone or anywhere without written permission.