If the Theia event was true, could it have formed a great deposit of diamonds out of the impact?? If that was the case, is it correct to suppose that somewhere hidden within the surface of the Earth there is a great diamond deposit resulting from that huge collision?
Well, no, due to a host of reasons:
Remember that Theia was a Mars-sized impactor. This means that the impact would have produced too much heat, enough to decompose any diamonds that would have formed.
Even if diamonds were formed, they would have been later been swept downwards by tectonic activity, to be distributed finely across the Earth.
Theia's diamond deposits might have gone on to the Moon. But don't get excited about mining the Moon for diamonds. According to new theories, the Moon may have been formed in less than a few hours, compared to the millions-of-years time required for planet formation. Such a quick formation would have been very likely to decimate lunar diamonds down to just lunar graphite, or even just carbon dust, distributed finely inside the Moon, due to the intense heat being produced.
Theia's impact was so powerful that it has created two continent-sized reservoirs of magma deep inside Earth's mantle. This means no diamond formation, as they would have been melted down into molten carbon, or have been broken down into the mantle.
No diamond mines for you, I am sorry.