Just prior to matan Torah Hashem told Moshe to inform Bnei Yisrael of the spiritual dividends that they would attain if they would accept the Torah: "If you listen to Me well and observe My covenant, you shall be for Me a segulah of all people, for the entire world is Mine" (Shemos 19:5). Rashi translates "segulah" as a beloved treasure, similar to the pasuk (Koheles 2:8), "v'segulas melachim" which refers to the treasure chest of kings which is hidden away. Rashi's explanation of the pasuk continues that Bnei Yisrael are Hashem's treasure, but we should not think that the reason they will be cherished is because they are the sole nation under His domain - "For the entire world is Mine" - Hashem has complete domain over all the nations. Hashem sees no significance in any nation other than Yisrael, thus it is evident that they are truly cherished as Hashem's "treasure chest."
Rav Wolbe (Shiurei Chumash, Yisro 19:5) elaborates on Rashi's explanation. He explains that Rashi was not making merely an etymological reference to the treasure chests of kings. There is an actual similarity between Bnei Yisrael and a king's treasures. Just as a king conceals his precious stones from the human eye, so too, if Bnei Yisrael observe Hashem's Torah and mitzvos they will be treasured and concealed i.e. granted access to penimiyus - the concealed areas of our world.
What is penimiyus? It is the ability to translate intellectual knowledge into practice. This is an area which demonstrates Bnei Yisrael's distinctiveness. All other nations gauge wisdom and success by their outward appearance. As long as the philosophy being espoused sounds impressive, the philosopher is awarded great accolades. It makes no difference whether the philosopher or anyone else acts in accordance with the newfound knowledge. For many, as long as the honor was attained the goal has been achieved.
The nineteenth century Danish philosopher Kierkegaard, the father of modern day philosophy, declared that he is afraid of only one thing: His theories would fall into the hands of a professor who will exploit it into a money-making venture instead of putting the knowledge into practice. His concern was well founded because that is exactly what happened! In contrast, let us appreciate Rav Yisrael Salanter, the father of the Mussar Movement, who lived at the same time as Kierkegaard. He revolutionized the study of mussar, and it is to his merit that mussar is studied daily in yeshivos worldwide. He is the direct impetus behind the millions of positive actions resulting from mussar study.
The other nations live in a world of chizoniyus with merely external trappings. By contrast, Bnei Yisrael were meant to live in a world of penimiyus. We must bear in mind that a diamond polisher might have all the knowledge needed to transform a mere stone into a brilliant jewel, but if he fails to put his knowledge into practice the stone will remain nearly worthless. Similarly, although Bnei Yisrael as a whole are inherently Hashem's treasure, any individual who wishes to shine must polish himself by transforming abstract morals into actions.