Diamonds are carbon. Plants take in CO2 and use the carbon. Chemically, could the right kind of plant have diamonds for berries, or is there some other limiting factor?
A plant could produce a diamond chemically by laying down carbon atoms in the right crystalline formation. Heat and pressure are for geological diamonds, they aren't relevant when you're dealing with atoms at a time. There is no reason for them to do this, evolution-wise, but since you're talking about genetic engineering that doesn't matter. Forming diamonds this way would only be limited by how much carbon the plant can get. They could strip hydrogen, oxygen, etc. from any organic compound, but it would probably be much more efficient to feed it carbon directly as either graphite or charcoal.
The problem is not with "diamond" but with "berry". From Wikipedia:
...a berry is a fleshy fruit produced from a single flower and containing one ovary.
Which a diamond definitely is not. But in terms of diamond trees with clusters of jewels, it's possible.
Quite possibly! Biology has the advantage of being able to micromanage what is going on chemically and mechanically. This often allows life to achieve results that humans currently achieve with huge chambers at incredible temperatures and pressures. With advances in genetic engineering we will become able to leverage this more and more.
We can now make synthetic diamonds quite easily using Chemical vapour deposition. Although it would be quite difficult for a plant to "handle" gaseous carbon due to its extreme temperature, the plant could instead liberate single carbon atoms at high energy. Of course this would be happening simultaneously all over the surface of the diamond, and the diamond would slowly grow, probably over years.
Note that the diamond itself would not be "alive"—it would certainly not have any reproductive capacity. Perhaps living cells could be embedded within it, though. It's possible that the enzymes and/or cells that surround the diamond during its formation would occasionally get trapped within it, particularly with "early prototypes" of the plant. I expect this would manifest as cloudiness of the diamonds. Conceivably such diamonds could become fashionable due to the knowledge that it is caused by living matter trapped inside, at which point genetic engineers might intentionally manipulate the process to create visible patterns within the diamonds.
Diamonds are considerably more than just carbon. According to this, there are believed to be four steps to the formation of diamonds:
- Bury carbon dioxide 100 miles into Earth.
- Heat to about 2,200 degrees Fahrenheit.
- Squeeze under pressure of 725,000 pounds per square inch.
- Quickly rush towards Earth’s surface to cool.
I'm fairly certain that plants combust a few degrees below 2,200 degrees Fahrenheit.
There are a few methods we've managed to produce synthetic diamonds with, but both involve high temperatures.
Definitely not naturally - for the simple reason this is completely impractical from evolutionary point of view.
Diamond requires a lot of energy to form. Even if the process was atom-by-atom, with the biology handily providing easily detached placeholders preventing the surface from oxidation (forming weak bonds with the diamond's carbon, then breaking them and replacing with more carbon atoms making the diamond grow) attaching each new atom would take a lot of energy; energy the plant must obtain on top of maintaining its own growth and life processes. Energy better spent on more useful endeavors like growing taller to outgrow competing plants, or producing more seeds to increase chance of finding fertile soil for them.
OTOH artificial species like this would be possible. I doubt it would resemble berry bushes as it would need a massive leaf system to acquire all the needed solar energy and carbon dioxide, a massive root system to provide water and nutrients to the massive leaf system, a "skeleton" to support both, and the diamonds would not be exposed to the air, instead growing inside some fruits that would prevent oxidation and contamination of the growth surface.
So - replace your diamond berries with diamond trees :)
Contrary to some of the answers here, there is no chemical reason why a plant couldn't manufacture diamonds (source: bachelor's degree in chemistry). The individual alkane bonds that make up diamond are nothing special, and living cells make and break that sort of bond all the time.
Artificial synthesis techniques work on atoms in bulk, whereas biological systems can synthesize molecules more or less atom-by-atom, and for this reason the constraints that make diamond hard to synthesize by bulk methods are mostly irrelevant to biochemistry.
As a very loose analogy, imagine you had a sack of Lego bricks and you wanted to join them all together into a solid block. In bulk chemistry terms this means continually shaking ("heating") and compressing the sack, and the bricks would join together to some extent, because the joined state is more space-efficient ("thermodynamically stable"), but it would take longer than the lifetime of the universe to get to a single solid block, unless you were shaking the bag really, really fast. As a living cell, you'd simply open the sack and put the bricks together one by one.
Biochemistry isn't magic, and there are thermodynamic costs to working in this organized way. In the above analogy, the living cell first needs to manufacture complicated specialist enzymes to grip and combine Lego bricks, and that background work will require a great deal of energy overall. The difference is, though, the energy isn't expended all at once, so it doesn't necessarily imply high temperatures or pressures.
In terms of evolutionary biology, there would need to be a good reason for the plant to evolve this feature (evolution doesn't waste energy). Even then, you could debate whether it's feasible at all in terms of energy landscapes; see e.g. the discussion about evolution and wheels.
If it were a genetically engineered feature, the question is not whether it could be done but only how hard it would be. Possibly the answer is "insanely hard", but that's the kind of question you can't answer until someone does it.
In A Deepness in the Sky, Vinge referred casually to strata bearing diamond forems. You might also think about diatoms which produce a cell wall of silica, literally glass.
I think it's plausible that microorganisms could produce structures of crystalline diamond or carbon fiber, in a variety of manners that real life produces inorganic shells in or around itself. Vinge used forems rather than more familiar diatoms, I think because the test (shell) is like a seashell, extruded around the cell. But diatoms produce glass in their cell walls, so you might imagine a mechanism where it is created in a completely enclosed chamber and then the outer skin is disposable leaving the protective shell to face the harsh environment.
Others have pointed out that nanotechnology or "life" could plausibility deposit crystals atom by atom, but I think carbon fiber is more realistic: look how carbon fiber is actually manufactured: start with an organic molecule that has a very common carbon hexagon backbone, and then removing all the extra atoms leaving only the carbon hexagons.
But I'd like to note that these structures won't be solid rock diamond crystals, but sparse filigree and thin walls, like diatom shells.
What might be the use of something like diatomaceous earth that's composed of diamond rather than silica? Obviously abrasives, but could a useful composite material be made?
If such a thing existed in nature, technological society would figure out how the nanotechnology works inside the cell, and apply the ideas to synthetic processes; or use selective breeding to produce algae that grow long fibers.
As a follow-up to knave's answer, for a diamond-fruit, I'm picturing a walnut-like fruit which has a fleshy outer layer, a hard middle layer, and the living part of the seed in the center.
The outer layer would prevent Hydrogen and Oxygen from binding to the Carbon while more layers of Carbon are deposited. A cut in the outer layer could potentially disrupt the carbon-laying process while the fruit is growing.
The diamond layer, like a walnut shell, would need a seam; a weak line along which the shell will split when the living inner part sprouts from the seed. The seam must be much weaker than diamond, as no seedling is going to have the ability to punch through even a thin layer of diamond. The shell might grow with several seams or just one, depending on the plant. It will also need a path for nutrients to enter the inside of the shell, which could be the seam or a hole. Depending on how the shell splits, you may end up with bowl-shaped diamonds...not ideal.
The inner part would need enough food for it to grow until the seedling applies enough force to split the shell along its seam. As such, it would probably be close to the size of a walnut, or even as large as a coconut.
One potential problem is that while diamond is hard, it is not flexible. Most plant matter is flexible because plants are always changing their shape. A diamond fruit would need to grow to its full size before it started growing a diamond layer.
One last thing. Diamonds are "valuable" because of market manipulation. There are much prettier stones and much rarer stones out there. Their scarcity in the jewel market is manufactured scarcity. Industrial diamonds are valuable because of their hardness, but they are common and can be manufactured already. The cost to develop a plant that grows diamonds would far outweigh the value gained from producing diamonds with plants, for both jewelry and industrial uses. It's more likely that an eccentric billionaire would fund development than that a diamond company would.
While it wouldn't be a berry I can think of a reason for a diamond to evolve: A plant with diamond needles inside it would be akin to being poisonous but it would be even harder for a creature to evolve resistance.
I doubt there is an evolutionary path that leads to this, though.