Law enforcement sources tell TMZ ... Tekashi refused to describe to police the type of jewelry stolen or even estimate their value, though he told us it was nearly $1 mil.

What's more ... we're told his driver would NOT fill out a lost property form for the stolen iPhone and would not tell cops if the tracking software had been activated. BTW, we're told Tekashi's GF has also stopped cooperating.

Refusing to divulge what was stolen makes it incredibly hard for cops to know exactly what they're looking for. But, we're told cops are still active on the case because, as we reported, surveillance video backs the rapper's claims.

As you know ... 6ix9ine says he was pistol whipped, kidnapped and robbed ... and maintains it was NOT a publicity stunt. As we reported ... Tekashi's bling is back around his neck, but they're not the OG pieces. He had copies made.