Well that's pretty cool, Katharine. You have an Aureus Magnus medal. It is worth its weight in gold. There are many designs for the front of the coin, but all of them (or most of them) have the spoked, fluted, and flowered cross design on the back. Very pretty!
It turns out that in 1955 a man named Werner Graul designed a series of gold medals (ducats) which he called Aureus Magnus (Latin for the great gold) as inflation-proof gold-based world currency. He wanted a 'currency-independent, single gold piece, which fulfilled the task of world current gold standard.' In the fall of 1957 he sent 10 designs to finance ministers around the globe, but his idea was ultimately rejected. Too bad. We might not be in our current financial mess the world had listened to Mr. Graul.
Today what we have is a beautiful series of gold medals, with cultural and historical themes. The definitive web site for these coins is (what else?) Aureus-Magnus.com, which displays the original designs and new designs in both silver and gold.
There are different sizes and weights of these coins, as outlined on this web page. Here is a summary. The Roman numeral appears at the center of the coin on the side with the cross.
1/2 DUCAT (S) 1.8 grams, 0.057 troy ounces
1 DUCAT (I) 3.5 grams, 0.112 ounces
2 1/2 DUCAT (IIS) 8.8 grams, 0.282 ounces
5 DUCAT (VI) 17.5 grams, 0.563 ounces
10 DUCAT (X) 35 grams, 1.125 ounces
20 DUCAT (XX) 70 grams, 2.250 ounces
30 DUCAT (XXX) 3.376 ounces
100 DUCAT (C) 11.254 ounces
You can find the current value of gold at kitco.com in US dollars per troy ounce. Multiply that number (it changes continuously) by the weight of your medal in troy ounces and subtract about 10 percent to account for the fact that the medals are not 100 percent pure gold. This gives an approximate retail price for the coin. If you are selling your medal to a dealer, he or she will pay substantially less than the retail price.