The gold lamb ducat shown is square, but others are round. They come from the old German Free City of Nuremburg in Bavaria. With a weigth of 7 grams, yours is probably the 2 ducat denomination. The 1 ducat weighs 3.5 grams.
NEVER CLEAN A COIN. CLEANING RUINS VALUE.
One catalog we have shows a round 2 ducat cataloging at $1500 US dollars in average circulated condition. But the same catalog shows ducats as low as $150. The 1 ducat coins have sold at professional auctions in the past for $1000 to $1600 US dollars, while one 2 ducat coin sold at auction in 2012 for $2000. All in all, we can average these observations and predict these approximate catalog values.
worn: $200 US dollars approximate catalog value
average circulated: $800
well preserved: $1200
worn: $400 US dollars approximate catalog value
average circulated: $1200
well preserved: $2400
As with all old coins like this, individual coins stand on their individual merits. Catalogs can give only very general guidance. If you have a nice looking specimen, seek out a knowledgeable collector or professional coin dealer for an in-person evaluation.
Our picture comes from Forum Ancient Coins and there, no doubt, you can find the experts you seek. For a good evaluation, provide a scan or photograph of your coin. Scanning coins usually gives better results than photographing them. If you do photograph, turn the flash off and use macro mode.
There are also a bunch of reputable dealers at VCoins.com who may be able to help.
Finally, there is a possibility that your coin is a counterfeit. This is the case with any valuable coin. Click on our Counterfeits category to see CoinQuest's small collection of fake coins, and what to do abou them.