What is the significance of planet Mercury in astrology? It is mostly known as the god of communication, merchants and medicine. Mercury is youthful and spritely, so its meaning is often confused with the star sign Gemini. As a result, Mercury is often wrongly attributed light-hearted, flighty traits. The planet is associated with early education, children, the media, and gossip. Other problems for the Mercurial type is a short attention span and the tendency towards being a jack-of-all-trades. Modern social media like TikTok and Twitter are very Mercurial.
However, “According to the Picatrix…successful students of magic generally have either Mercury or Jupiter as the strongest planet in their nativities.”  Mercury is also associated with Hermes and Thoth both gods of wisdom. So there is a deeper meaning behind this traditional god of communication.
Mercury rules both Gemini and Virgo. The latter sign possesses more serious characteristics, such as duty and healing. This is probably where we get the medicinal/chemistry connection as well. In alchemical art, Mercury is one of the most used symbols as it is one of the three primes. (The others are sulfur and salt)
Mercury’s symbol “ represents the mind, as well as a state that could transcend death… Mercury was known as quicksilver.. and able to shift between liquid and solid states. Therefore, in alchemy, Mercury was believed to shift between life and death.” ~ prepscholar
So Mercury can shape-shift and make connections. It is also shamanic and able to navigate between dimensions. Within the symbol of Mercury, you can see the glyph of Venus, which is associated with the pentagram. So magic is common to both of these inner planets.
Sometimes Mercury and Venus are interchangeable. The word hermaphrodite, often a symbol for Mercury, contains them both! (Hermes = Mercury. Aphrodite = Venus.) Word magic, mind-control, propaganda and spells all go hand in hand! Positively Mercury brings quick-wit, manual dexterity and cleverness.
The role Mercury plays in our horoscope describes communication style. It also shows what areas of life we are most interested in and where we are the most mentally agile. Manual skills, technical ability and handicrafts are ruled by Mercury. Businesses that involve advertising, sales and marketing are great for mercurial types as it is the planet of merchants.
Mercury’s metal is quicksilver “the shining globules of this liquid metal form and reform so quickly, as fast as thinking. The metal mercury is the one element that one normally sees in the three states of matter – as the fluorescent lamp overhead in the classroom, as the liquid in the thermometer and as calamine the skin lotion; as Hermes was the one deity who could come and go through the three worlds.” 
Mercury can also drive you mad. The substance was used in the making of top hats in Victorian times and of course highly toxic. “Likewise the metal mercury amalgamates: different metals can be brought together by dissolving them in mercury, it is a solvent for metals.”  Mercury fillings or amalgams are made from the same metal. Interestingly, these tooth fillings are placed in the mouth and close to the mind, which Mercury also rules.
Mercury as significator
Literary men, philosophers, mathematicians, astrologers, merchants, secretaries, diviners, sculpturers, poets, orators, advocates, schoolmasters, stationers, printers, exchangers of money, attorneys, commissioners, clerks, artificers (designer), accomptants (experts) solicitors, sometimes thieves, prattling muddy ministers, grammarians, tailors, carriers, messengers, footmen, usurers (money lenders).
Places & Colours
Tradesmen’s shops, markets, fairs, schools, common halls, bowling-alleys, ordinaries (commons), tennis courts. Mixed and new colours, grey mixed with sky-colour, or consisting of many colours mixed in one.
Herbs attributed to Mercury are known by the mixed colour of the flower; they love sandy barren places; they bear their seed in husks or cobs, they smell rarely or subtlety, and have principle relation to the tongue, brain, lungs or memory; they dispel wind and comfort the animal spirits and open obstructions. the walnut and walnut tree; the eldertree, adders tongue, dragonwort, aniseed, marjoram. Herbs that are used for the muses and divination, such as vervain. (Interesting!)
Manners when well placed
A man of a subtle and political brain, intellect, and using much eloquence in his speech, a searcher into all kinds of mysteries and learning, sharp and witty, learning almost anything without a teacher; ambitious of being exquisite in every science, desirous naturally of travel and seeing foreign parts: curious in the search of any occult knowledge; able by his own genius to produce wonders; given to divination and the more secret knowledge; if he turn merchant, no man exceeds him in a way of trade or invention of new ways to obtain wealth.
Manners when badly placed
A troublesome wit, a kind of frenetic man, his tongue and pen against every man, a great liar, boaster, prattler, busybody, false, a tale-carrier, given to wicked arts, as necromancy and such like ungodly knowledges; easy of belief, an ass or very idiot, cheating and thieving everywhere; a news-monger, pretending all manner of knowledge, but guilty of no true or solid learning; a trifler; a mere frantic fellow; easily perverted, constant in nothing but idle words and bragging.