Albert Barnes Commentary Job 28:6

"The stones thereof are the place of sapphires, And it hath dust of gold." — Job 28:6 (ASV)

The stones of it are the place - Among the stones of the earth sapphires are found. “The situation of the sapphire is in alluvial soil, in the vicinity of rocks, belonging to the secondary floetz trap formation, and embedded in gneiss.” Jameson. “The sapphire occurs in considerable abundance in the granitic alluvium of Matura and Saffragam, in Ceylon.” Davy.

Sapphires - Compare the note at (Isaiah 54:11). The sapphire is a precious stone, usually of a blue color, though it is sometimes yellow, red, violet, green, or white. In hardness it is inferior to the diamond only:

“In unrolled tufts, flowers purpled, blue and white,
Like sapphire, pearl, in rich embroidery.”
Shakespeare

“He tinctures rubies with their rosy hue,
And on the sapphire spreads a heavenly blue.”
Blackmore

The mineral is, next to the diamond, the most valuable of the precious stones. The most highly prized varieties are the crimson and carmine red; these are the “Oriental ruby” of the traveler, and next to the diamond are the most valuable jewels previously discovered.

The blue varieties—the sapphire of the jeweler—are next in value to the red. The yellow varieties—the “Oriental Topaz” of the jeweler—are of less value than the blue or true sapphire. Edinburgh Encyclopedia, article “Mineralogy.”

And it has dust of gold - Margin, or “gold ore.” Literally, “The dusts of gold are in it.” Gold is often found in the form of dust. It is obtained by washing it from the sand, and passing it over a fleece of wool, to which the gold adheres.