Your great granddad had a nice coin. I am assuming it is a 5 franc piece, with '5 F' on the reverse. These coins contain 0.723 troy ounces of silver.
Most of the dates in this series, including Robert's 1867 date, are common. They catalog roughly for $10 to $15 US dollars in worn condition, rise to the $20 to $30 range in average circulated condition (like our picture). Here is a more data with more specific detail:
average circulated: $25
well preserved: $50
fully uncirculate: $90
5 francs dated 1865 and 1866: $500 in average circulated, $1200 in fully uncirculated condition
5 francs dated 1867 with a dot after the F on the reverse catalog at $70 in average circulated and $275 in fully uncirculated condition
5 francs with the 1868/67 overdate catalog at $150 in average circulated and $700 in fully uncirculated condition
5 francs before 1869 with signature just below Leopold's neck and not near the rim of the coin: $1000 and up (see below)
Be sure you understand that these 'catalog values' are not actual values. Look at our Terminology page for this interpretation.
If you have a 5 franc coin dated 1865, 1866, 1867, or 1868, and there is a small-lettered name directly beneath Leopold's neck, you have a rarity worth at least $1000 in average circulated condition.