Isolated, the Jack of Diamonds represents the postman or, better said, the messenger. It also symbolizes a young soldier or an unfaithful servant (assistant, employee); sometimes a young blond man, traitor and of bad morals.
If the consultant is a young man, it is the announcement of new lovers.
If it is a young girl or a single woman, it is the certainty that, soon, a suitor will make himself known; he will be blond and pretty boy.
If, on the contrary, the consultant is married, he will soon learn of a somebody’s else marriage; but this card, surrounded by it, has many and varied meanings.

The Jack of Diamonds with a
Spade ♠: Treachery, infidelity.
Heart ♥: Certainty of marriage and news of birth.
Diamond ♦: Young risk-taking man, adventurer of misconduct.
Club ♣: Somebody will ask to borrow money from you.

Having on the left a
Spade ♠: A dark man you have every reason to fear.
Heart ♥: Love. Letter coming soon.
Diamond ♦: Null card.
Club ♣: Money that will be lost. Theft.

EXCEPTIONS.
The Jack of Diamonds, with the
King of Diamonds: News from a young adventurous man.
Between two sevens: News of a pregnancy.
Touching the Ten of Diamonds: Distant journey. (The other card touching it becomes null and void.)
Touching the ace of Clubs: News of an inheritance. (The other card touching it becomes null and void.)
Between two face cards: deceit, treachery, infidelity.
Between two aces: The Jack of Diamonds says nothing; he is absolutely silent.

Young dark man, fake, mean and treacherous.
While declaring his friendship, he meditates inwardly on betraying you; his morals are light, his associates deplorable.
Very talkative, he easily discloses to anyone what you have entrusted to him in good faith; even the most serious things he has sworn not to say. He does not hinder himself in any way to perjure himself, as long as it can attract him a profit, however small it may be.

This Jack having on the right a
Spade ♠: Bad acquaintance. – False friend to oust.
Heart ♥: Somebody will want to date you and get into a relationship with you; just say “no” to him. – Bad words from a young brown man.
Diamond ♦: Slander in the city.
Club ♣: Abuse of trust. – Flatterer to be feared.

Having on the left a
Spade ♠: Unscrupulous. – A young dark man will try to seduce you. He is not as powerful as the King of spades. If you recognise him, you should have no trouble of resisting him.
Heart ♥: A joyful meeting that will go wrong towards the end of the evening.
Club ♣: Jealousy. – Discomforts.

EXCEPTIONS.
Between two cards of the same value: Bad intentions in a business of family meeting.
Between Aces: Your enemy will go to jail.
With the Queen of Spades: Criminal attempt; the other card touching the Jack of Spades is void; it does not speak.
Near the Eight of Spades: Violent death of a friend; the other card touching the Jack of Spades is null; it does not speak.

The King of Diamonds sometimes symbolizes a soldier, or more generally – a man in a dangerous occupation, exposed to risks of injury or violent death (policeman, firefighter, roof-tiler, aircraft pilot and similar professions with elevated risks). For a young woman or a divorced woman, the isolated card means a proposal for marriage from a man in a dangerous occupation.
It also represents a blond man, self-important, haughty, fearsome, deceitful; in a word, a bad man.
It is one of the cards whose meaning depends mostly on the cards surrounding it.
So the number of its exceptions, i.e. special cards that give it a different meaning, is quite varied, as you will see below.

This card announces, having on the right a
Spade ♠: Naughty, quarrelsome man, whom you should be wary of.
Heart ♥: A beginning of a relationship with someone, who is not good for you. If the consultant is married: new acquaintance not to be frequented.
Diamond ♦: Flatterer of which we have everything to fear.
Club ♣: Somebody will try to steal from you.

Having on the left a
Spade ♠: Obstacle in your projects by a blond man.
Heart ♥: Serious suitor. If the consultant is old or married: loving and honest man.
Diamond ♦: Quarrel.
Club ♣: Advantageous offer by a blond man.

EXCEPTIONS.
The King of diamonds placed between two face cards, whatever they may be, means: Disunity, confusion.
With the Jack of DiamondS: News from a young man in a dangerous profession.
Between two SpadeS: Rivalry.
With the Queen of HeartS: Love of a soldier. For married or elderly people: Upcoming acquaintance of an honest man, military.
Having an ace on the right, the usual meaning of the card on the left is nil. Instead, this announces, regardless of the colour of the card on the left – upcoming trip away related to some danger or a man in dangerous business.
The King of Diamonds, between two diamonds, announces the visit of a man from out of town.