Unfortunately there is not a feature built into Mint for automatically tracking items like silver, I apologize for any inconvenience this may cause. Unless you have some sort of other online account tracking this for you that you can link to Mint, the only work around I can offer is to manually enter the value and manually adjust it at your discretion. To do this, go to Accounts at the very top of the page, click the Other tab, (if you already have something else in this section, next click the orange +Add Other Property button), choose Other Property, and select the best option from the drop down menu. Enter the worth, then you can come back here to adjust it as needed.
It's not an elegant solution, but it's the best one available. However, I have tagged this post as a feature request so that the developers can see it for future consideration. If you do happen to have an online account already tracking these commodities for you but we don't yet support it, you can request support be added with the Contact Mint form. We'll usually look at adding a new financial institution once we have at least 5 individual users requst it, but not all can be supported due to various security incompatibilites. https://www.mint.com/help/#more-help
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