The most common and most likely answer to this crossword clue is the 8 plus 6 letter word PRECIOUS METALS
I've seen this crossword clue in The Daily Telegraph
P
R
E
C
I
O
U
S
M
E
T
A
L
S
More clues leading to the result PRECIOUS METALS
Buddy explains!

The straight part
We know that "They are very valuable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are very valuable = precious metals.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.