Emerald is an ancient stone with references to it scattered all throughout the bible. 

Exodus 28:20, 39:13, Ezekiel 28:13, Revelations 4:3, 21:18  Emerald is a green variety of beryl. One of the earliest known source of emerald were mines located near the Red Sea in Egypt. There is evidence that these mines were in operation as early as 1650 BC (the time that the Hebrew people would have been in Egypt). Later these mines became known as Cleopatra’s Mines, who was quite fond of emeralds and was reported to wear them to enhance her beauty. 

Emerald is the third stone in the first row of the breastplate of Aaron. The first mention of stones in the Bible (Exodus 28:17-20) has to do with the stones in Aaron's breastplate. Aaron was Moses' brother and led the tribe of the high priests. His breastplate was adorned with twelve stones and is described in the Bible as the breastplate of judgment or decision. "You shall put settings of stones in it, four rows of stones: The first row shall be a sardius, a topaz, and an emerald ; this shall be the first row; the second row shall be a turquoise , a sapphire, and a diamond; the third row, a jacinth, an agate, and an amethyst; and the fourth row, a beryl, an onyx, and a jasper." 

There are 12 different stones on Aaron's breast. each stone represents one of the tribe of Judah. Emerald being the third stone is also believed to be representative of the tribe of Judah. The tribe of Judah stands out among the twelve tribes because of it's associations with the house of David, the southern kingdom of Judah, and its capital in Jerusalem. The blood line and lineage of  David is also the blood line and lineage from which Jesus was born Matthew 1:1–17.