Detailed Explanation

Background:

We have explained in detail that a finder may not acquire ownership of a "find" so long as the owner does not despair from retrieving the lost article. Instead, should the finder encounter the article before the owner despairs, he or she must attempt to fulfill the Mitzvah of Hashavat Aveidah, pick it up, safeguard it, publicize the find and ensure that it is returned to the correct owner.

An owner will not despair if there is reasonable hope of retrieving the article. Losing an identifiable article amidst a society of Hashavat Aveidah observers lends itself to the owner's reasonable hopes of retrieval. As such, an owner of an identifiable article found amidst a society in which even most of the population is Hashavat Aveidah observant does not despair [see Issue 50].
 
It is important to note that finding an identifiable article in an area frequented mostly by non-Hashavat Aveidah observant individuals is not a free license to keep the article. Although the owner will probably despair upon discovering the loss, and as such, the finder need not publicize the find, if the owner does actually produce identifying details, it is noble and morally correct for the finder to return the article to its original owner. It is a Mitzvah to follow this "noble rout" of being "straight and good".

Although court maintains no legal right to penalize one for opting not to do so, the finder is strongly encouraged to return the article under such conditions. Note: A destitute finder is morally exempt from returning a find under such conditions to an otherwise affluent original owner. Instead, the original owner has a Mitzvah to be "straight and good" [Devarim 6: 18].  He should allow the finder to keep the find without a conscience. [Choshen Mishpat 259: 5].

Emerald Cabochons and Belgian Chocolates implicates the following three laws.

1. Identifying features include unique size, shape, color, weight, and packaging to name a few [Choshen Mishpat 259:2, 262:6].

2. Similarly, the intentional positioning and setting of an article may serve as an identifying feature if it is unique [Choshen Mishpat 259:2, 262:3] Note: An expert is able to distinguish identifying features that may not be discernible to the amateur's eye.

3. Providing identifying features on one article can only indicate ownership of an adjacent "lost article" if it is clear that the two articles were lost together [Choshen Mishpat 262: 20].  Otherwise, the original owner must produce separate identifying features for each article.

Belgian Chocolates
The ever-growing family of Pralines, Fresh Cream, Truffles, Genache, Gianduja, Marzipan, etc, and their innumerable packaging styles and a selection of finer manufacturers leave an incredible selection of literally hundreds of variables. Clearly, a box of Belgian Chocolates would pass as an identifiable article.

The position of the find though on the showcase could only serve as an identifying feature if it was an unexpected position- something unique and out of the ordinary.
 
Emerald Cabochons
Similarly, while perhaps lacking the numerous details of faceted cut colorless diamonds, the smooth cut Emerald Cabochons, indeed bear countless identifying features.

Let us familiarize ourselves with a  bit of pertinent information regarding gem cutting and emeralds cabochons in particular. [The following information is gleaned from the Britannica Dictionary, and various Wikipedia articles on Emeralds, Sapphires Cabochons, Facet Cuttings]. 
 
"The ideal product of facet cutting is a gemstone that displays a pleasing balance of internal reflections of light known as brilliance, strong and colorful dispersion which is commonly referred to as "fire" and brightly colored flashes of reflected light known as scintillation. Typically, transparent to translucent stones are faceted.

The angles used for each facet play a crucial role in the outcome of a gem. While the general facet arrangement of a particular gemstone cut may appear the same in any given gem material, the angles of each facet must be adjusted carefully to maximize the optical performance. The angles used will vary based on the refractive index of the gem material. When light passes through a gemstone and strikes a polished facet, the minimum angle possible for the facet to reflect the light back into the gemstone is called the critical angle. If the ray of light strikes a surface lower than this angle, it will leave the gem material instead of reflecting through the gem as brilliance.

A cabochon however is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones. The usual shape for cutting cabochons is an ellipse.

Nevertheless, emeralds, like all colored gemstones, are graded using four basic parameters, Color, Cut, Clarity and Crystal or transparency. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emerald, crystal is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.

Color: Scientifically speaking, color is divided into three components: hue, saturation and tone. Yellow and blue, the hues found adjacent to green on the spectral color wheel, are the normal secondary hues found in emerald. Emeralds occur in hues ranging from yellow-green to blue-green.

Clarity: Emerald tends to have numerous inclusions and surface breaking fissures. Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated, "oiled", to enhance the apparent clarity. Eye-clean stones of a vivid primary green hue (as described above) with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone command the highest prices. This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes."

Application:
 
Emerald Cabochons are not standard. They have an innumerable amount of identifying features ranging from hue, saturation, tone, weight, cut - shape, clarity number of inclusions,  transparency, degree of treatment  etc.

In addition, the Belgian chocolates have an innumerable number of identifying features. 

However, it is reasonable to assume that the positioning of the emerald and the chocolate on Mark's showcase were nothing unique.
 
Having visited scrupulous individuals, the owner remains hopeful of retrieval. Mark may not keep his find. He must publicize the find without revealing too many identifying features, [Choshen Mishpat 267:4] whereby leaving room for the truthful owner  to prove ownership thereof by produce the accurate identifying features.  Sending an email to the salespersons that walked through his door that day [Choshen Mishpat 262: 4] informing them that someone "left something behind in my office today" would warrant a good publicizing effort. [See further issues for more detailed discussion on the topic of effective publicity.]

Even if the original owner would have despaired, if the salesman subsequently produced the accurate identifying features, Mark would fulfill the Mitzvah of being straight and good by returning his find to the original owner.

Presumably, there was no indication that the Emerald and Chocolates were left together. Thus, Mark must require the original owner to produce separate identifying features for the Emerald as well as for the Belgian Chocolates.