Prima Materia is the formless base from which all matter is derived.
An anology, although an imperfect one, could conceivably be made to the difference between stem cells and actual organ or nerve cells. Just as a stem cell can develop into any of the various types of actual cells found in the human body, so Prima Materia can serve as the source for any a specific type of elemental matter.
I’d recommend that you read “The Red Lion” by Maria Szepes though, If you do read it, pay attention to the passage of what happens when the main character of the book actually succeeds in an alchemical experiment – it might prove enlightening.
“Szepes tells the story of the unhappy Hans Burgner, a miller’s son born in the 16th century. After the death of his weak father and of a likewise miserable but beloved teacher, he becomes afraid of the unavoidable death of all living things. Driven by a monomania fed by persistent rumors of an Elixir of Immortality, he becomes an apprentice of a mysterious physician and alchemist. However, instead of listening to the Alchemist’s compassionate counsel and warnings, Burgner is driven by feverish greed to murder him; in this way, he acquires Elixir while he is still spiritually unprepared, and is cursed thereby. This is the starting-point of a journey through the centuries: while Burgner can physically die, the Elixir enables him to retain the full memory of his previous lives as he repeatedly reincarnates into a variety of different circumstances. It also bestows upon him a profound spiritual sensitivity. Several times he attempts the Great Transmutation in order to deliver himself from his self-imposed curse. Hans Burgner is refined through his various incarnations. Against the backdrop of the last five centuries of European history, he undergoes dramatic personal development: beginning as a spiritually unawakened (and even infamous) character, he matures spiritually through the various challenges he is led to confront. He is first an initiate and Aspirant, eventually attaining the perfection of human personality which characterizes the Magus, or spiritual Adept.”