Here is another picture. Thank you

I don't think its rockwool. looks more like a compressed peat starter.
What website did you get the pictures from?

I was thinking peat, too. I'm not sure what the white stuff is. Maybe it's some sort of treatment they give the plugs to deter algae.

The is the link for the first picture ( http://www.lighting.philips.com/main/application_areas/horticultural/case_studies/case_study_deliscious_nederland.wpd ), while this is the link to the company's website where they show a video at the bottom of page of their production process ( http://www.deliscious.eu/en/home-2 ). The second picture I got after searching 'japan plant factory' on Google and looking at the images tab.

Thanks in advance for taking time to answer my question.