Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Technically, all these coins are still legal tender at face value, though some are far more valuable today for their numismatic value, and for gold and silver coins, their precious metal value. From 1965 to 1970 the Kennedy half dollar was the only circulating coin with any silver content, which was removed in 1971 and replaced with cupronickel. However, since 1992, the U.S. Mint has produced special Silver Proof Sets in addition to the regular yearly proof sets with silver dimes, quarters, and half dollars in place of the standard copper-nickel versions. In addition, an experimental $4.00 (Stella) coin was also minted in 1879, but never placed into circulation, and is properly considered to be a pattern rather than an actual coin denomination.
What are gold and silver coins valuable for now adays?