Q: I would like to find some free silver coins with my new metal detector, but I wonder if that might be a violation of Deut 7:25
A: The greater context of Deut 7:25 is this: Israelites entering the Land of Promise, driving out and/or slaughtering the pagans before them, and discovering their statues/idols made of gold and silver used as objects of worship (i.e., graven images). The Israelites were told not to take the objects in as statues, but were told to melt them down. Now, you can interpret that to mean, after melting the statues down, they simply tossed the liquefied precious metal onto the ground to be forgotten forever. However, I'm somewhat doubtful that is what they were commanded to do (Does anyone actually know what they did?). They likely turned it into bullion and eventually used it in coins and/or jewelry. It should be somewhat obvious that the graven statues would not/could not be given to the Tabernacle while still in the state of a graven image and maybe even not after being melted down. I don't know what they did in practice, BUT I highly doubt they turned it into compost or sidewalk material... nor do I think they donated it to their neighboring enemies.
I'm curious how this entire passage is perceived when read in Hebrew. The way sentences are laid out in English sometimes confuses meaning or the ancient original intent. For example, in the KJV it looks like they were being told to melt down the statues and told not to even keep the resultant metal, but we're not told what they were told to do with it after it was melted down... which causes me to think we're missing something in translation.
The graven image issue tends to make me think that many of our coins with carved images of people will likely be melted down and re-minted with non-idolizing images. Even so, for example, I don't know anyone who idolizes or worships the queen on the Canadian Maple or the goddess on the American Silver Eagle, though I'm sure there are some who do. Currently, it is illegal to melt those coins down in their respective countries because they are considered national currencies, but when those nations cease to exist... I suspect many of them will be melted down and turned into something else. Whatever is done in the future restored United Kingdom of Israel... will be done there and outside the domain or controls of other kingdoms/nations. I can only guess how other national currencies will be treated. Maybe they'll be traded for freshly mined, never minted bullion. Who knows???
To answer the question more specifically though: there are coins out there that do not have any graven image on them, and of course I see no risk in violating Deut 7:25 in that case... and maybe not in any case, even with Queen Elizabeth on them, for example. She isn't/wasn't being worshiped by pagans who you are being commanded to slaughter nor have you invaded the Land of Promise to find her being worshiped there. Using your metal detector to find something precious is your prerogative. What you do with what you find is a whole other matter. =)
P.S. In case you're wondering, this was a question asked by a FB friend today. I'm not sure of the intent or hypothetical nature of the question. It did seem well-crafted and I'm still waiting to see if this person actually recently obtained a metal detector for such a purpose as finding silver coins.