Sapphires, a variety of corundum stones, have a Mohs hardness scale value of nine. This is the same hardness as rubies and second only to diamonds as the hardest minerals known to man. Because of the difficulty in attempting to engrave such a hard stone, the gem in the second row of the breastplate was likely a lapis lazuli. With a Mohs hardness of 5.5 it is much easier to engrave than a sapphire.
"The ancients gave the name of sapphire also to our lapis-lazuli, which is likewise a blue stone, often speckled with shining pyrites which give it the appearance of being sprinkled with gold dust . . . it is an opaque substance easily engraved" (Catholic Encyclopedia article on Precious Stones in the Bible).
"The stone cannot have been our sapphire, for both Theophrastus and Pliny describe the sapphirus as a stone with golden spots, thus showing that they meant the lapis-lazuli, which is often spotted with particles of pyrites having a golden sheen.
"This stone was named chesbet by the Egyptians, and was highly prized by them . . . " (Curious Lore of Precious Stones, page 293).
This gemstone was often paid in tribute to ancient Egypt, being obtained from some of the oldest mines in the world. The gemstone, made in the image of the pagan goddess of truth, was also worn around the neck by the Egyptian high priest (ibid. pages 119, 229, 293).
Marco Polo, in 1271 A.D., is believed to have visited the Asiatic mines that produced the lapis lazuli (Diamonds, Pearls and Precious Stones, page 92).
The second row, middle stone in the priest's breastplate in the Bible is likely not a sapphire for the reasons stated above. However, a true sapphire was likely used to adorn Lucifer since God himself decorated him. Additionally, since the Eternal will create the New Jerusalem, which will come down from heaven, like a bride (Revelation 21:2), there is little doubt that he will use sapphires for one of its foundations.
The cure for feeling melancholy, and for a reoccurring fever, was thought to come through a Lapis lazuli gemstone. Ancient Romans and Greeks believed the stone had medicinal applications. They were known to pulverize it and use it to make a tonic (Curious Lore of Precious Stones, pages 92 - 93, 370).