What a gorgeous coin, Philip! Jose (Josephus) I was king of Portugal and Algarves (Port et Alg) from 1750 to 1777. He issued these beautiful gold coins in colonial Brazil in four denominations: 800, 1600, 3200, and 6400 reis. Here are the stats on diameter and gold content:
800 REIS: 14 mm diameter, 0.053 troy ounces gold
1600 REIS: 20 mm diameter, 0.106 troy ounces gold
3200 REIS: 25 mm diameter, 0.211 troy ounces gold
6400 REIS: 30 mm diameter, 0.422 troy ounces gold
There were similar gold coins issued in homeland Portugal. This page applies to the Brazilian coins, minted in Bahia and Rio de Janeiro, Brazil, with mint marks B and R under Jose's profile. Coins without mint marks were minted in Lisbon and use escudo denominations. Values of the Lisbon coins are similar to the Brazilian coins.
As with all gold coins, you should start an evaluation with the basic gold value. Go to kitco.com to get the current gold price per troy ounce. (Note: the word 'troy' is understood when speaking of precious metals.) Then multiply that price by the weight of gold for your coin, listed above. For instance, at this writing, gold is $1618 US dollars per troy ounce. The basic gold value of a 6400 reis is then $1618 x 0.422 = $682.
To the basic gold value we now add value due to collector appeal. These are old and beautiful coins, so examples in good shape garner a lot of collector appeal. Here are some typical values:
add $100 if circulated
add $500 if fully uncirculated
add $200 if circulated
add $1500 if fully uncirculated
add $2000 if circulated
add $8000 if fully uncirculated
add $400 if circulated
add $1000 if fully uncirculated
With his request to CoinQuest, Phillip sent a picture of his coin. Philip's coin has no mint mark, so it was minted in Portugal and not Brazil, but it is suspect. It may be a fake. In addition to the subtle differences in the pattern, which are easily seen in the side-by-side comparison below, the date is the main problem. No Josephus coins were minted in 1778. By that time Jose was no longer King of Portugal and genuine coins dated 1778 showed the portraits of Maria and Pedro, who took over in 1777.