You may have a taxable capital gain.
You have a capital gain if you sold the property for more than it's cost basis. For inherited property, cost basis is the fair market value on the date the previous owner died.
As long as the property was held more than one year (combined between you and your grandparent) then this is a long-term capital gain which is taxed at 15% for most people and 20% for high income taxpayers. However, if audited, you must be able to prove the cost basis you claim. The IRS does not have to give you credit for any cost basis that you can't prove.
You report the sale of the coins as capital gains income on schedule D, this is in the "Sale of assets and other things" section of the income page. Enter the date inherited, fair market value at the time, date sold, proceeds, and any other questions you are asked.
You need to make a diligent effort to document and prove your cost basis, otherwise the entire amount will be taxable (zero cost basis). If these were bulk silver coins sold for melt value (weight x daily price for silver) then you can look up the price of silver on the day your grandmother died, and calculate the approximate weight from the number and size of the coins using internet references, or you can use the same figure the coin shop used. (If they called it 5,000 grams of silver, you can use that same figure.). Melt value would at least be a minimum value that you could reasonably prove and use as your cost basis.
If these were special or rare coins that had a value more than melt value, you would need an appraisal to document any cost basis that was higher than melt value. I don't know how you would go about doing that if you never had them appraised before.
