Why does Nephi speak of an abundance of precious ores in 2 Nephi 5:15 in the Book of Mormon, and then say precious things were not to be found upon the land in verse 16?
The verses in the Book of Mormon that you refer to are—
15 And I did teach my people to build buildings, and to work in all manner of wood, and of iron, and of copper, and of brass, and of steel, and of gold, and of silver, and of precious ores which were in great abundance.
16 And I, Nephi, did build a temple; and I did construct it after the manner of the temple of Solomon save it were not built of so many precious things; for they were not to be found upon the land, wherefore, it could not be built like unto Solomon’s temple. But the manner of the construction was like unto the temple of Solomon; and the workmanship thereof was exceedingly fine.
Solomon’s temple was ornate almost beyond belief. Not only was every part of the temple, both inside and out, covered with gold, but the appointments and accouterments were extremely prolific and of the very finest of material. Josephus, in describing Solomon’s temple, recounts in part—
“He [Solomon] also made ten thousand candlesticks, according to the command of Moses…The king also made pouring vessels, in number eighty thousand, and a hundred thousand golden vials, and twice as many silver vials; of golden dishes, in order therein to offer kneaded fine flour at the altar, there were eighty thousand, and twice as many of sliver. Of large basins also, wherein they mixed fine flour with oil, sixty thousand of gold, and twice as many of silver….the sacramental garments which belong to the high priest, with the long robes, and the oracle, and the precious stones, were a thousand…he also made ten thousand sacerdotal garments of fine linen, with purple girdles, for every priest; and two hundred thousand trumpets, according to the command of Moses; also two hundred thousand garments of fine linen, for the singers that were Levites; and he made musical instruments, and such as were invented for singing of hymns, called Nabloe and Cinyrae [psalteries and harps,] which were made of electrum, [the finest brass] forty thousand.”
These were the many precious things; for they were not to be found upon the land, to which Nephi referred.