Gold sovereigns and half sovereigns are gorgeous coins. All of them have portraits of reigning British monarchs on the front, and usually a crest or St. George on horseback slaying a dragon on the back. A few, like this one, has a wreath around the name of its country of origin: Australia.

All these coins are valuable, especially the half sovereigns. Value is a combination of basic gold value and collector premium, as explained below. Here is pertinent info:

HALF SOVEREIGNS: 19 mm diameter, 0.1177 troy ounces gold
collector premium circulated: $100 US dollars
collector premium well preserved: $4000
collector premium fully uncirculated: $10000
half sovereigns dated 1855 are very rare. A typical value is $20000 for a circulated coin

SOVEREIGNS: 22 mm diameter, 0.2353 troy ounces gold
collector premium circulated: $50 US dollars
collector premium well preserved: $200
collector premium fully uncirculated: $2000
sovereigns dated 1857, 1859, and 1862 carry collector premiums of about $2500 in well preserved condition
sovereigns dated 1858 and 1860 carry collector premiums of about $5000 in well preserved condition
sovereigns dated 1855 carry collector premiums of about $10000 in well preserved condition

To find an approximate value of your specific coin, first multiply its gold content by the current price of gold. This gives the basic value. For instance, the basic value of a half sovereign at today's gold price ($1474 US dollars per troy ounce, look it up for current value: kitco.com) is 0.1177 x 1474 = $173 US dollars. Second, estimate the collector premium using the listings above and add it to the basic gold value. For a circulated half sovereign, that would be $173 + $100 = $273.

You can see that some of these coins are worth a ton of money. If you have such a coin, congratulations! You should consult with a knowledgeable coin dealer or collector and have your coin authenticated, graded, and encapsulated by a third party numismatic (coin collecting) service. Use PCGS, NGC, ICG, or ANACS for this purpose. Do not use other services.