Precious Coral

Both precious red and black corals are bottom-dwelling sessile marine branched animals that have been harvested at depths between 60 and 20,000 feet [Ref 1, Ref 2]. They have provided lapidarists and other artisans with materials they use to create extraordinarily beautiful jewelry and objects of art, as witnessed by Figures 1 and 2. In this Blog, I’ll describe the physical properties and the sources of the colors of the corals that underlie their use in jewelry making and in carving fine objects of art.

As shown in Figure 1, the Chinese Qing Dynasty red coral sculpture, featuring four carved figures of Buddha, capitalizes on the flowing branched nature of the coral to produce a beautiful and graceful carving. The flowing shapes of the branches

of black coral are an interesting contrast to the disciplined shapes of the gold fittings set with gems, as seen in Figure 2.

Figure 1. Chinese Qing Dynasty carving in branched red coral with four figures of Buddha, circa 1862-1874 [Ref 3].

Figure 2. Black coral necklace with accents of green and pink tourmaline, blue topaz, amethyst, citrine, and fire opal set in 18-carat gold [Ref 4].

PROPERTIES OF PRECIOUS RED AND BLACK CORALS

Gemstones of red and black coral comprise the hard tissue core or skeleton which supports the soft tissues of the organisms as shown in Figures 3-5.

Figure 3. A red coral *in situ* with white soft tissue supported by the red coral tree. The soft tissue contains the polyps, the feeding organ of the coral and tissues connecting them [Ref 5].

Figure 4. Sections of branches of the red coral skeleton. The section on the left has been bleached. The furrows on the surface of the skeleton contain the part of the soft tissue which distributes nutrients [Ref 6].

Figure 5. A black coral (*Cirrhipathes sp*.) with yellow soft tissue surrounding the hard tissue core is shown on the left and the same specimen with its soft tissue removed is shown on the right [Ref 7].

PROPERTIES OF PRECIOUS RED AND BLACK CORALS

Gemstones of red and black coral comprise the hard tissue core or skeleton which supports the soft tissues of the organisms as shown in Figures 3-5. The physical and chemical properties of each coral underlie its usage in jewelry and art objects.

Physical and Chemical Properties of Precious Red Coral

The hard tissue of the skeleton of red coral is comprised of aggregated nanometer-sized crystals of magnesium-rich calcite {(Ca.Mn)CO₃} which are arranged in rings separated by an annular layer of organic matrix containing glycoproteins and glycosaminoglycans[Ref 8]. The annular arrangement of the mineral and organic matrix is shown in the cross-section of a stem in Figure. The organic matrix is highlighted by a purple stain and the layer of magnesium-rich calcite is unstained [Ref 9]. The axial region rich in magnesian calcite is indicate by the letter

A and the spaces occupied by the longitudinal canals, the regions of soft tissue which transport nutrients are labeled by the letter B.

Figure 6. Cross-section of a stem of red coral in which the soft tissue has been stained purple [Ref 10].

Pigmentation in Precious Red Coral

The red pigment in precious red coral has been determined to be canthaxanthin, a member of the family of retinoids which are ubiquitous in red-colored organisms [Ref 11]. Its location in the outer soft tissue and organic matrix is demonstrated by its red color in Figures 7 and 8. The range of hues of the red color of the coral as a gemstone is shown in Reference 12.

Figure 7. The localization of the red pigment canthanxanthin in the organic matrix is demonstrated by the lack of color in the sclerites (spicules) [Ref 10].

Figure 8. Further demonstration that the red pigment is present in the organic matrix [Ref 13].

Mechanical Properties of Precious Red Coral [Ref 14]

The values of Mohs Hardness in the range 3-4 for red coral are comparable to that of 3 for black coral [Ref 14]. With its axial core of magnesian calcite, red coral is brittle with an irregular or splintery fracture, and cannot be bent to assume complex shapes as can be done with black coral. Natural shapes and those obtained by carving are similar to those in black coral jewelry and art objects as can be seen in comparing the jewelry and art objects in Figures 9-17 to those in Figures 18-26.

RED CORAL JEWELRY AND ART OBJECTS

Figure 9. Necklace incorporating branches of red coral [Ref 15].

Figure 10. Red coral bracelet with gold fittings [Ref 16].

Figure 11. Italian good-luck-horn pendant
[Ref 17].

Figure 12. Goddess Flora carved in red coral [Ref 18].

Figure 13. Red coral necklace with silver beads, Yemeni [Ref 19].

Figure 14. Figure of Mulan carved in red coral [Ref 20, Ref 21].

Figure15. Carved flower pendant set in 14 carat gold [Ref 22].

Figure 16. Red coral rose set in sterling silver ring [Ref 23].

Figure 17. Carved red coral snuff bottle. Chinese [Ref 24].

Physical and Chemical Properties of Black Coral

Some black corals are branched, while others have long and straight stems or spirally twisted stems, as shown in Figures 18 and 19. The stem or branch of the black coral skeleton is not composed of a core of magnesian-rich calcite as in red coral, but consists of laminated composites comprised primarily of protein and chitin fibrils [Ref 24]. The growth of the diameter of a stem or branch proceeds by accretion of layers, as shown by the polished cross section of a stem in Figure 18 [Ref 25]. The black color seen throughout the stem is due to the melanin [Ref 25], a black pigment widely distributed in both the animal and plant kingdoms [Ref 26].

Figure 18. Branched black coral [Ref 27].

Figure 19. Spiral black coral with and without the external tissues which resides on the central skeleton [Ref 28].

Figure 20. Transverse polished section of black coral. (*Leiopathes species*)
showing layers of chitin after removal of proteins. The fine structure of
concentric rings is evident. The diameter of the stem is ~ 7 mm {Ref 29].

Mechanical Properties of Black Coral

To compensate for the lack of stiffness, due to the absence of a skeletal core of rigid magnesium, as in red coral layers, the chitin fibrils [Ref 30] comprising the strong part of the skeleton, is spirally wound in the concentric layers [Ref 30] comprising the stem and branches as shown in Figures 8-10. Such a winding scheme works to prevent kinking of the stem due to bending and twisting of the stem due to torsional forces. This scheme compensates for the lack of a stiff mineral axis as is present in red coral. Values of the mechanical measures of the strength of the skeleton of black coral are summarized in Table I taken from Ref 30.

Figure 19. View of two layers of chitin fibrils in two layers the stem of black coral.
The fibrils are wound in spiral fashion about the axis of the stem. The scale bar
is 10 microns (0.0010 cm) [Ref 30].

Figure 20. Sketch showing different spiral patterns in layers of chitin fibrils in two black coral species [Ref 30].

TABLE I MECHANICAl PROPERTIES OF SKLETONS OF TWO SPECIES OF BLACK CORAL (Antipathians)[Ref30]

PROPERTIES	ANTIPATHES FIORDENSIS	ANTIPATHES SALIX
Mohs Hardness	3	3
Micro-indentation Hardnesshardness along the long axis (pounds/mm²)	26,000 lb/in²	31,663 lb/in²
Micro-indentation hardness along a diameter (pounds/mm²)	28,968 lb/in²	32,516 lb/in²
Extensibility (%)	7.37	3.84

The Mohs hardness of 3, of black coral, is comparable to the range of values 3-4 of red coral, so that expectations of resistance of both to wear, particularly scratching are similar. Values of micro-indentation hardness in the range of 26,000 to 32,516 lb/in² are approximately one-half the value of 58,064lb/in² estimated for the rhombohedral surface of calcite estimated from Figure 1. These values of Mohs and micro-indentation hardness suggest care in wearing black coral jewelry. Jewelry such as pendants, earrings, and necklaces of this coral will have longer scratch-free life expectancy.

The large room-temperature values of the extensibility of the two black coral species of 3.845% and 7.37%, especially augmented by the thermoelasticity, [Ref 31], of the coral at higher temperatures underlie the shaping of jewelry with intricate shapes. The softness of black coral underlies the use of carved shapes in jewelry. Working the coral at an elevated temperature would further enable attaining intricate shapes with finer carved relief.