The distinctive characteristic, or great beauty of a true pearl (natural and cultivated) is its lustre (or orient). Lustre is a subdued iridescence, as opposed to the glittering brilliance of the diamond, and is the most important consideration when selecting a pearl. Lustre should be bright - never dull. Without lustre it does not rank as a gem pearl, no matter how perfect its form or beautiful its colour. Because South Sea pearls have a thick creamy nacre, they are noted for their rich silky lustre, and are thus valued most highly.