The injectors are perhapse the second most creepy thing seen in a kindergarten following the holes. We know the injectors are part of gem production but exactly how they work and the exact nature of gem production are still mysterious. To truly understand the process we must know WHAT the injector actually injects into the ground.
First lets review what we know about injectors and how they relate to gem production.
Injectors start a process that is ongoing and independent of the injector once started. We can be sure of this because the number of injectors seen at both kindergartens is much smaller then the hole count indicating that an injector is able to move on after it injects, which is why they have legs too.
The developing gem draws energy from the local area creating a local dead-zone, this means the injector is not the main source of energy for a developing gem, so the injector dose not need to hold or inject large amounts of energy.
Because gems emerge from the ground with fully developed minds and bodies the initial injection event must impart all of the hereditary information nessary for a gem to fully develop. We don't know exactly how much a gem knows immediatly upon emerging, Amethysts case seems to indicate that they will 'imprint' on anything around them as they seek friends and kinship, but they may not know any actual gem history, technical knowlege or the norms of gem culture.
Now for the direct quotation evidence
In Keeping it together Pearl states that injectors 'plant gems in the crust of the Earth' and the visual shows a small point of light leaving the drill tip and growing as it absorbs swirls of light. Notably the initially light spark and the absorbed light look similar and we are not shown a gem at any point.
In How gems are made Amethyst states 'they put the machines on Earth and shot this junk into the dirt and then BOOM'. The term junk is clearly crude but is sugjestive that what is injected is not an object but something fluidic.
In Gem Drill Peridot states that the rocks being drilled through are periditite which is 'made of the same stuff as Peridots' and clearly expresses an emotional affinity with the mineral, this means that gems are LITERALLY normal minerals just infused with energy and sentient thought. The term is not a euphemism for the shapes of the gems or any superficial resemblence to minerals, they ARE minerals. Furthermore new gems memories begin at the point when they first emerge and take form.
Now putting all this together the simplest theory is that the injector simply puts a small seed gem in the ground that grows in size as it draws in energy until the gem is mature and can emerge. This would also be the most consistent explination with real mineralogy, a small crystal will grow as material precipitates out of solution onto it.
But this theory has many flaws. First the only known source of small gems is the shattering of larger gems, and the resulting shards would not make good candidates for growing a new gem from. Shards clearly retain lots of memories of their past lives and no amount of passive growth in the ground should make them forget this life, which should leave the newly born gem with traumatic 'past life' memmories which have never been observed. Second gem shards can form bodies all be it incomplete and have an almost irrisistable urge to do so, this should cause them to litterally claw their way out of the ground immediatly rather then patiently grow, indeed that was basically how the cluster was supposed to function.
If gems grow from smaller seed gems then the TYPE of gem would be completly dictated by the type of seed, a shard of amethyst would be needed to make a new amethyst, and a shard of peridot to make a new peridot etc etc. This would imply that thouse running the kindergarten would have full control over the types of gems created. But we know that the Beta kindergarten made both Jasper and a Carnelian, if the process is so easily controled why are multiple gem types being made at the same time, it strongly sugjests that homeworld can't actually control the process that fully. Note also that both known Beta gems are red/orange in color matching the local rock, while Amethysts colors match her kindergarten. This sugjests that the native rock is dictating the color and type of gems being produced which would be inconsistent with simply burrying a shard in the ground.
Lastly we have the logistical problem, if shards can be turned into new gems then homeworld in creating the cluster was forfiting the oportunity to make huge numbers of new gems with which to fight the war, and as they clearly tried to incresse the production of new gems it seems odd to waste the raw material used to make them, even taking it off Earth for injecting somewhere else would make more sense. You would think that Peridot would have mentioned this wasted potential, in an area she had expertise in no less, to YD too when arguing against the cluster.
So if the injector dose not inject a small seed gem then what is it injecting, I belive it is injecting information and energy not material. Basically the essense of gem, the same energy that gems themselves absorb from the environment to grow. This energy pattern carries the heriditary information of gems and it enters pre-existing mineral crystals in the ground and turns them into developing gems much like a zygote or 'gemgote' as I call it. This is consistent with all the lines of logical evidence seen so far and it explains the virus like apearance of the injectors, because a virus only transmits heredity information while the host provides all the material.
Under this theory their is a single universal gem essense that all injectors inject and that all gems are made from, the type of mineral that this energy enters is all that creates the wide range of gem types we have seen, so gems are truly one species not hundreds of seperate lineages. I belive that even the diamonds are made this way, they are simply incredibly rare and particularly powerful gems, this explains how Pink Diamond has so few planets, she was a relativly new diamond.
Now the next question, if the injector is primarily injecting information/essense can it make infinite numbers of gems because it just 'makes' this essense onboard. I think not, the gem essense is an energy pattern carrying enormous information and while the injector can store it and inject it they can't create it or EDIT it in any way so it is depleted from the injector as it dose it's job. The reasoning is as follows, IF the information pattern could be edited then the diamond authority would easily be able to make completly docile and maliable followers, which they cleary want btw, but everything we have seen indicates that they have no power to do this and instead use every available means of social control such as propaganda, punishment and disinformation to control the gempire. And if the pattern can not be edited then it can't be created from scratch either because that would nessesitate the ability to edit it. Lastly the fact that injectors are just abandoned as 'junk' sugjests their is nothing particularly special happening inside that would be worth recovering once they run out of injectables.
So where dose homeworld collect this gem essense from.
THEY HARVEST IT
Yes, gem harvest, the long forshaddowed process Peridot is so afraid of is an integral part of making new gems, that is why she as a 'master kindergartener' was familar with the process and assumed it was going to be done to her, despite shattering being the most common way that gems seem to die in battle or as a means of capitol punishment. Peridot has seen harvesting done before, may have even done it under orders to condemed gems, and make no mistake about it the process is essentially fatal to the gem, draining their energy and likely leaving the gem inert or perhapse usable only as one of the many 'utility' gems we have seen that 'power' things.
This contrasts with other theories from prominent Youtubers and Reddit commentor about gem harvest which belive it is simply shattering of gems to make shards for use in the cluster, or a process that primarilly makes the utility power gems. Neither of these ideas explain Peridots connection, this is why harvest must be connected to kindergartening and the injectors.
Another small piece of evidence, what is on the main characters shirt in this manga http://imgur.com/a/ZNSaq I hadn't even noticed this untill after I had the theory developed and it may just be a coincidence but it could be an intentional clue or homage.
The whole choice of the term harvest now makes sense too. The agricultural nature of a cycle is now clear, first gems are planted, then they are harvested and then the result of the harvest makes possible the next planting like seedcorn. While I may be wrong about the gems energy and essense being the means of harvesting rather then shattering, I am very firm on this anwer to the opening question.
TLDR Injectors are injecting whatever a gem harvest harvests.