The Islamic Dinar is a specific weight of 22k gold (917.) equivalent to 4.25grams.
The Islamic Dirham is a specific weight of pure silver equivalent to 3.0 grams.
Umar Ibn al-Khattab established the known standard relationship between them based on their weights: "7 dinars must be equivalent to 10 dirhams."
"The Revelation undertook to mention them and attached many judgements to them, for example zakat, marriage, and hudud, etc., therefore within the Revelation they have to have a reality and specific measure for assessment [of zakat, etc.] upon which its judgements may be based rather than on the non-shari'i [other coins].
Know that there is consensus [ijma] since the beginning of Islam and the age of the Companions and the Followers that the dirham of the shari'ah is that of which ten weigh seven mithqals [weight of the dinar] of gold. . . The weight of a mithqal of gold is seventy-two grains of barley, so that the dirham which is seven-tenths of it is fifty and two-fifths grains. All these measurements are firmly established by consensus." Ibn Khaldun, Al-Muqaddimah
How are the Islamic dinar used?
1.- The Islamic Dinar can be used to save because they are wealth in themselves.
2.- They are used to pay zakat and dowry as they are requisite within Islamic Law.
3.- They are used to buy and sell since they are a legitimate medium of exchange.