Will this angel's sweet voice loosen your tongue? (i.e. Perhaps you will tell the late President Franklin?)

If you try to develop a sound fantasy-currency mind the following details.
There always is the currency and the coinage and the two are not necessarily the same. A currency may be based on a weight of silver but perhaps there is no coin representing this weight. Or the basic unit of the currency may be a certain coin but this coin is no longer in use, replaced by a multiple of its weight which is easier to change into a popular foreign currency.
Secondly people do not call coins by their proper name but use colloquial terms. A coin may be worth a four crowns but it is called an "Angel" in common parlance.
This may sound complicated. For the purpose of illustration I have chose the Elizabethan English system to show you how a mid-renaissance currency and coinage looked like:

The basis of the currency system is quite simple:
1 pound (£) is one troy pound in silver. About 370 g.
It is made up of 20 schillings (s). Each shilling is about 18.5 g silver.
1 shilling in turn is 12 pence (d) Each penny (!) weighing about 1.5 g silver. (This is the weight of 1 Euro-cent coin and by the way about the weight of the Carolingian denier, of which the penny is a distant descendent. The d in the abbreviation stands for "d"enier, which in turn is nothing else but the Roman denarius. Brand names do go a long way!)

The coinage was made up of ...

Metal	Coin	Worth
Fine Gold	Sovereign	30 s
Gold	Sovereign/Pound	20 s
Gold	R(o)yal	15 s
Gold	Half Laurel	10 s
Gold	Angel	10 s
Gold	Noble	6s 8 d
Gold	Half Angel	5s 6 d
Gold/Silver	Crown	5s
Gold/Silver	Half Crown	2s 6d
Silver	Shilling	12 d = 1 s
Silver	Sixpence	6 d
Silver	Groat	4 d
Silver	Threepence	3 d
Silver	Twopence-farthing	2 1/4 d
Silver	Threehalfpence	1 1/2 d
Silver	Penny	1 d
Silver	Threefarthing	1 d
Silver	Halfpenny	1/2 d
Silver/copper	Farthing	1/4 d

Gold was struck in different standards of fineness. Standard gold 23 carats 3½ grains. Crown gold 22 carats.
In 1603 silver was restored to the standard before Henry VIII debasement. i.e. 11oz 2dwt of silver to 18dwt alloy. Both Henry VIII and Edward VI minted Farthings, Elizabeth did not. Henry VIII minted a silver Testoon worth 1 Shilling reduced in value from July 1551 to 9 Pence then 6 Pence.

Silver "Portcullis " money minted in 1600 specifically for the Company of Merchants trading to the East Indies. These "Testerns" minted to an exact equivalent weight in silver to the Spanish "Real". This was a coinage specifically for foreign trade because the Indian merchant were accustomed to the Spanish coins.

8	Testerns	equal silver to	8	Reals
4	Testerns		4	Reals
2	Testerns		2	Reals
1	Testern		1	Real

The Sovereign or Double R(o)yal, was a gold coin with a value of twenty shillings. It was first produced by Henry VIII (1485-1509) and continued through 1662. The coin had a diameter of about 4,25 cm and weighted about 11.2 g in fine gold. The name probably stems from the picture showing the king enthroned in full splendor. Under Elizabeth I sovereigns from standard gold were struck, worth 20 s or 1 pound. Others from fine gold had a value of about 30 s and weighted 15.5 g.

The Ryal was a gold coin with a value of ten shillings introduced by Henry VII (1485-1509) to replace the Noble (which had been valued at 6s8d). The coin was unpopular and was replaced by the Angel. Mary (1553-1554) and Elizabeth I (1558-1603) struck a fifteen shilling ryal.

The Half Laurel, Half Sovereign or Double Crown had a value of ten shillings. The double crown was first produced by Henry VIII (1485-1509) and continued through 1662.

The Angel was a gold coin introduced by Edward IV (1461-1470) with a value of 6s8d to replace the unpopular ten shilling ryal. The name for the coin comes from the obverse design showing St. Michael spearing a dragon. Under Edward VI (1547-1553) the value of the coin was increased to ten shillings. It had a diameter of 2,7 cm and weighted about 5.6 g in gold.

The Noble was introduced under Edward III. It was unpopular and depicted the king standing in a ship.

The Half Angel was a gold coin introduced during the restoration of Henry VI (October 1470-April 1471) with a value of 3s4d. In 1526 under Henry VIII the value was increased to 3s9d. Production of the coin continued through James I, with the value raised to 5s6d in 1612. The final issue of half angel coins was minted in 1619.

The Crown, valued at five shillings, dates back to Henry VII (1485-1509), when it was made of gold. Edward VI (1547-1553) struck the first silver crowns in 1551-1553, which were over twice the size of his gold crowns. The gold crowns had a diameter of a little more than 1 cm and weight of a little less than 3 g. The large silver crown must have weighted around 40 g with a diameter of up to 3 cm. These coins were inspired by the large silver coins of Austria and Spain. Both countries had access to vast deposits of silver. The first in the mountains of Tyrol the latter in the Americas. These coins were called Thaler, which is, where the Dollar gets his name from.

The Halfcrown, valued at 2s6d, was introduced as a gold coin under Henry VIII (1509-1547). Production of the hammered gold half crown continued into the reign of James I who minted gold halfcrowns (1603-1619) and also produced a larger sized silver halfcrown (1603-1625).

The silver Shilling dates back to Henry VII (1485-1509) when it was known as the "testoon." By the early Seventeenth Century it became a important coin with several issues minted. The testoon bore the portrait of the ruling monarch in profile. This was an artistic innovation inspired by Italian renaissance coins. It weighted around 18.5 g and had a diameter of about 3 cm but was rather thick to bear the strong relief of the portrait.

The silver Sixpence dates back to Edward VI (1547-1553). The first milled sixpence were produced by Elizabeth I during 1561-1571.

The silver Fourpence, originally called a Groat, goes back to Edward I (1272-1307). It is often thought the fourpence coin was part of the Maundy series. That is, part of a series of specially produced coins that were not made for circulation but rather were ceremonial. Maundy coins were special products for the ruler to distribute to the poor on Maundy Sunday; a ceremonial tradition that still continues in England. 6 g silver, 2.7 cm diameter.

The silver Threepence was first issued under Edward VI (1547-1553) with milled coins being produced by Elizabeth I during 1561-1564.

The silver Twopence, originally called a half groat, was first produced in 1351 under Edward III (1327-1377). Diameter 2 cm, weight 3 g. Usually written "tupence".

The silver Penny appear to have been first introduced into England during the reign of King Offa in 757. For centuries it was the only coin struck in the realm with some 70 different mints producing the coin during the rule of William the Conqueror (1066-1087). No other denomination was produced in England until the short lived 20 pence coin under Henry III (1216-1272). It weighted about 1.5 g and had a diameter of about 1.8 cm.

The silver Halfpence dates from the period of Henry I (1100-1135) and Henry III (1216-1272) but the series did not become a regular issue until Edward I (1272-1307). With a silver contents of 0.7 g this coin was rather tiny.

Recently a silver Farthing from the reign of Henry III (1216-1272) has been uncovered, but the series did not become a regular issue until Edward I (1271-1307). The last silver farthings were minted under Edward VI (1547-1553). The series was then suspended as the coins were so small they were difficult to mint and were unpopular with the public as they were frequently lost. When the series was renewed by James I the farthing was a larger coin minted in copper, or occasionally tin. However these hammer struck coins were not royal issues but produced by individuals who obtained a royal license to mint them such as Lady Harrington and the Duke of Lennox. The practice of issuing royal licenses to mint farthings continued under Charles I. During this period farthings averaged about 17 mm in diameter and about 0.58 g in weight.