They are primarily eight precious ones.
Fill in the X s with appropriate letters to reveal all the 8 Precious Metals.
Each X represents a letter.
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The solution is:
1: Silver, 2: Iridium, 3: Gold, 4: Ruthenium, 5: Rhodium, 6: Osmium, 7: Platinum, 8: Palladium
Of the eight precious metals, Gold is the only one with four characters, so it is fairly clear that Gold is 3. Afterwards, Of the two with six characters, silver is the only one with an L in the third position, making Silver 1 and leaving osmium to be 6. Iridium is the only one that starts with I so it's in position 2, then rhodium is the only word that fits with the h and o in 5, finally palladium matches with the D in 8 and then platinum is the last remaining precious metal.