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The most common misidentification of a cameo's material seen on eBay and similar sites concerns the paler shades of red coral, sometimes referred to as 'angel skin coral', and queen conch shell. A recent search of eBay listings for items with the terms 'angel skin coral cameo' in the title returned results that were overwhelmingly conch shell cameos or coral cameos in shades befitting only a very sunburned angel. The blurring of the distinction is not new and quite possibly not unintentional. In 1882, the firm Francati & Santamaria, preeminent purveyors of both raw materials for cameo cutting and gem engraving and of finished work, with establishments in England, Italy and France, advertised among their wares 'Pink Coral Shells', meaning Strombus gigas, or, as it is called now, Aliger gigas, the queen conch.
True angel skin pieces are not common and the boundary separating this shade from its more sanguine relatives is highly subjective. The three pieces shown below are all genuine coral. Whether the color of each qualifies as 'angel skin' is up to the reader.
The material most often mistaken for coral is queen conch shell, a mollusk native to the waters of the Caribbean and around the US state of Florida. These large heavy shells were used as ballast by ships returning from delivering their cargo to the Americas. One can imagine how quickly cameo artisans would have taken to the thickness, sturdiness and pale shades of this material. Its resemblance to light pink coral (and the possibility of charging tourists coral prices) was surely not lost on them. It could regularly yield rough sections of predator damage-free material much larger than all but the rarest specimens of coral. The bases of the protective protuberances, 'horns', are especially thick.
Coral and seashells grow in two very different manners. Before they settle down for good in the apartment complex that is a coral colony, each little piece is a free swimming polyp. Apples don't fall far from the tree, and baby coral polyps aren't released far from the parent colony, but they may latch on at any spot on any colony in the neighborhood. The color of the little hard shell each produces is genetically determined. Red coral can vary in color from white to deep crimson. There is nothing compelling a polyp to join up with neighbors who share its place on the spectrum. Hence the jumbled, swirled coloration so often seen.
By contrast, conch shell is produced by a single organism through a process of accretion at the shell's lip of layered material specialized to the creature's survival needs: a thick outer layer to withstand the environment outside the shell; a smooth inner layer to provide a comfortable home for the mollusk within. Queen conch shells produce inner linings that range from nearly white to hot pink and an outer layer that is white. Most conch cameos are carved with a white figure or scene on a pink ground, but shells can be so large and thick, it is also possible to cut out sections that can be used in reverse, with the white layer for ground. These layers of color are the hallmark of conch.
Coral almost never looks like conch shell; conch shell, on the other hand, can do a very good imitation of coral. If one had only the photograph to the left to go by, an identification of coral would seem certain. 
And yet the front of the piece, shown to the right, displays the discrete layers of white and pink that confirm it as conch.
The higher value placed on coral easily leads us into temptation, into believing a conch piece is coral. Like most coral cameos, conch pieces are usually fairly thick and generally flat on back, although some are naturally a bit rippled. Both are nearly opaque, requiring a very strong light behind to show any translucence. As discussed later in the article, conch sometimes has pits and furrows on the back, just as many coral cameos do. Conch specimens with clearly delineated layers of white and pink are easily identified; those with less well organized coloration and those that are all a single solid pink or white are the teases.
As can be seen above in the photo showing the interior of a queen conch shell, the inner layer may have a high degree of variability in coloration, as well as fluctuations in the thickness of this layer. The artisan begins by slicing straight through the curved shell wall to remove a section. The natural irregularities of the shell would become fully apparent only as the piece was worked. The back of this Anchor of Hope piece bears some resemblance to the variegated look of some coral cameos; the clean white figure on the front, standing out crisply from its less decisive background, makes the determination of conch shell an easy one.
This lovely cameo of Diana, in shades of blush pink to white, could understandably be mistaken for angel skin coral, particularly from the front. However, the solid pink back tells a different story.
A bacchante draped in grape vine was a favorite subject of both coral and conch shell cutters. The thickness of these materials allowed for much higher relief than could be achieved in helmet shell or most hardstone (agate). Some of the most beautiful pieces we see are of the type shown here. They are also some of the most frequently misidentified as angel skin coral. Seen only from the front, this lady's composition is somewhat ambiguous; coral may produce small sections of color arranged in layers.
The more solid white of the back further confirms the layered nature of the piece. The side view clinches the matter: the piece is conch shell.
Before she was freed to become a beautiful brooch, she would have looked something like this: 
In the example to the left, the conch shell carving goes from white to pink, then back to white.
The reader will notice that the pink of conch shell is not the same as the more salmon color of undyed coral. Angel skin coral is a shade of light red flirting with yellow; conch pink is cozying up to its neighbor in the other direction, blue. Another difference, not always detectable from photographs alone, is in the texture of the surfaces. Coral can take a very high polish, and pieces are typically buffed to a high degree of gloss over all surfaces, both figure and ground. Some of the most convincing artificial coral pieces are glass or Celluloid. The white layer of conch has a bisque china quality which artisans generally did not polish. The pink layer is like glazed porcelain and would have required little or no extra polishing to make it gleam.
The bracelet to the left is angel skin coral. The beads exhibit the haphazard arrangement of color characteristic of its kind.
It would be a forgivable sin (until you finish reading this) to think the necklace below is also coral.
Closer examination of individual beads, a few representative examples of which are shown here, tells the true story. The stratification of the color indicates a molluscan origin; the color suggests not conch but some other thick walled shell, probably helmet shell. The lip of even a medium sized specimen provides suitably thick material for cutting beads.
Conch shell beads are less common than ones cut from helmet shell. Again, it is highly tempting to see them as angel skin coral. But, the distinctive, slightly purplish, shade of pink identifies the beads to the left as conch shell.
The substance of the very pale, monochromatic cameo below, going by the color of the front alone, is ambiguous. Conch shell pieces can be solid pink or solid white. However, the glossy surface of the figure and the orange streaks on the back reveal it as coral.
On the other hand, while this piece is very similar in color, and was offered for sale as coral, the distinctly delineated strata show it is shell, like the beads shown above, probably from the Cassis family.
This coral Ceres exhibits a slight layering of color, a tinge of a richer shade of orange on her back and shoulder, and at places in her hair. However, once again the shade is a yellow pink, not the blue pink of conch. The glossy surfaces of the front and the almost wood grain appearance of the back settle the matter.
It is tempting to see a piece like this Diana as white coral. However, the matte finish of the entire piece, its relatively large size (almost 3 cm in height) and its solid, very pale pink back allow it to be identified as conch shell. On the cheek and neck, this piece also exhibits the faint striations not infrequently seen on conch cameos. While having the appearance of fine cracks or scratches that have become discolored, a fingernail will not catch in them and no amount of cleaning diminishes their appearance. Running nearly straight and in parallel, they probably mark growth increments and so are further evidence that the material is molusk-produced rather than coral.
As seen in the photos of the backs of a couple of the pieces shown at the beginning of the article, coral frequently has blackened pits, sometimes even small tunnels, made by marine predators with a taste for coral. As long as they do not mar the front, their presence can be seen as an asset: makers of artificial cameos meant to look like coral rarely go so far as to add black spots for verisimilitude. These black marks, although not proof positive by themselves, are evidence supporting an identification of genuine coral. (N.b., Celluloid pieces that have been subjected to the hot needle test will have small pits, occasionally blackened.)
Anyone who has picked up battered shells on Miami Beach will know that they, too, often show the gouges made by predators. Conch cameos carved with the white outer layer for background occasionally show such marks, another temptation to see conch as coral. Despite the appearance of the back, this Flora is, once again, conch shell.
This cameo, with its overall polished shine, looks very like angel skin coral at first glance. Closer inspection reveals several features that identify it as shell. Visible on the front are the darker, nearly parallel lines seen on some conch pieces. While in the photograph of the front the cameo appears to be a yellowish shade with tinges of orange, the back shows unmistakably as conch shell pink and white. The saddle shape of the bezel gives the final photographic clue; a query to the seller confirmed the piece is slightly curved, something not seen in coral pieces.
