No, diamond is relatively rare.
There is no pronoun that means 'abundant source'. Some nouns that mean 'abundant source' are mine or well. Some adjectives that mean 'abundant source' are much and rife.
Now Romania hasn't an abundant source of energy.
The most abundant source would be in glacial form as frozen water, although the most abundant and accessible source is groundwater.
One source of diamond is volcanic pipes.
Coal is the most abundant in the USA
Romania has energy resources but not so abundant.
Solar.
The sun
Solar.
Bolivia and Chile
yes
Geothermal