It was coral. Among the many raw materials that King Hiram provided to Solomon for building the Temple, I Kings 10:11 records that he "brought from [the land of] Ophir much almog wood" (see also II Chronicles 9:10). Rashi and Radak comment that almog wood refers to קוראיי"ל [coral]. It appears to be classified as "wood" because it grows from the ocean floor in the shape of a tree (see Rashi to Rosh Hashanah 23a).
|Phoenician trading ship (phoenicianresearch.weebly.com)
King Solomon used this coral to make paving stones for the Temple (I Kings 10:12). It is hard to imagine how Hiram could have provided corals large enough to make paving stones because even though archeologists have documented a heavy coral trade around the Mediterranean region, the actual pieces of coral that have been found are quite small — on the scale of beads and other small ornamental pieces. Nonetheless, from the account in Kings it appears that the massive delivery of coral from Hiram to Solomon was unique in its scale, for the verse (ibid.) states that never in the past had such an amount of coral been harvested, nor will it be in the future.
The Gemara (Rosh Hashanah 23a) describes the technique that Hiram's divers would likely have used to harvest coral on such a large scale. A big empty ship was floated to the area containing the coral and then the ship was filled with sand until it sank into the water over the reef. [Rashi (ad loc.) notes that the water was shallow enough so that the top of the ship was visible above the water.] Divers then descended into the water with strong ropes and tied the corals to the ship. They then began emptying the sand from inside the ship until it floated up to the surface, uprooting the corals from the sea bed. This operation required tremendous manpower, as it took twelve thousand men working for six months (according to one opinion) to complete it. All of this effort was well worth it, because coral traded for twice its weight in silver.
|Greek men playing lyres (wikimedia)
The Mishnah (Keilim 17:13) states as a general rule that anything made from a material that comes from the sea is impervious to tumah. This includes the bones or skin of sea creatures, as well as any wood, plant, or grasses that grow in the sea (see Tiferes Yisrael §103 ad loc. and Rambam, Hilchos Keilim 1:3), and thus it would appear that coral is impervious to tumah. The only caveat is that if the sea material is attached to some other material that can contract tumah, the entire utensil becomes susceptible to tumah (see the Mishnah loc. cit. as well as Keilim 13:6). So to preserve the special status of these utensils they must be made entirely of materials that cannot contract tumah.
With instruments made of wood and metal there is always the concern that, despite careful safeguards, the instruments may wind up contracting tumah. Using coral ensures that this would never happen, thus adding a layer of protection and sanctity to the Temple service.