Diamondoids are found in natural hydrocarbons such as oil, gas, and condensates and are composed of carbon from biological sources. This is contrary to the carbon found in abiogenic sources, which are believed to originate from non-biological processes deep within the Earth. The presence of diamondoids in natural hydrocarbons is evidence against an abiogenic source for several reasons:

1. Biological Source Indication: Diamondoids are more thermally stable than most other hydrocarbons, and their molecular structure suggests a biological origin. The depletion of 13C in oil fields and diamondoids is further evidence, as the chlorophyll cycle in plants favors the retention of 12C, leading to a characteristic isotopic signature in biologically derived carbon.

2. Thermal Stability and Maturation: Diamondoids' high thermal stability means they can survive the high temperatures and pressures associated with the maturation of organic matter in sedimentary basins. This survival indicates that they are not products of abiogenic processes, which would require extreme conditions that are not typically found in the Earth's crust where hydrocarbons are formed.

3. Structural Complexity: The rigid fused-ring structure of diamondoids is more consistent with biological precursors than with abiogenic synthesis. Abiogenic processes are not known to produce such complex and specific molecular structures under the conditions prevalent in the Earth's crust.

4. Occurrence in Fossil Fuels: Diamondoids are reported to occur in petroleum fluids and other fossil materials, further linking them to biological processes. Their presence in these contexts suggests that they are part of the organic matter that has been transformed into hydrocarbons over geological timescales.

In conclusion, the presence of diamondoids in natural hydrocarbons, along with their isotopic composition, thermal stability, and structural complexity, provides compelling evidence that they are of biological origin. This contradicts the abiogenic theory of hydrocarbon formation, which posits that hydrocarbons are formed from non-biological carbon sources deep within the Earth. For more detailed information, you can refer to sources such as the article on the "Abiogenic petroleum origin" from Wikipedia [1] or the research paper on "Diamondoids: Occurrence in fossil fuels, applications in petroleum exploration" [2].

[1] https://en.wikipedia.org/wiki/Abiogenic_petroleum_origin [2] https://www.researchgate.net/publication/262871645_Diamondoids_Occurrence_in_fossil_fuels_applications_in_petroleum_exploration_and_fouling_in_petroleum_production_A_review_paper