Rob Kardashian: Blac Chyna's My $35k Diamond Girl

Rob Kardashian's Valentine's gift to Blac Chyna wasn't just romantic and thoughtful -- it was pricey and literally dripping in diamonds ... TMZ has learned.

We found out Tonia Calderon is the artist behind the portrait Rob bought for BC. As we reported, the piece includes lyrics from John Legend's song, "The Beginning."

Sources connected to Rob tell us he dropped $35,000 to commission the artwork ... which took Calderon 2 days to complete. If you're wondering why a 7 foot by 7 foot painting costs $35k -- we're told Calderon covered all the lyrics with diamond dust, which is the residue left after diamonds get cut.

Diamond dust goes for $50 per ounce.

Our sources say Rob and Tonia met through mutual friends a while ago, and he was impressed by her work. She's made similar pieces for Lady Gaga, Dr. Dre and Diddy ... who ordered a Biggie portrait.

We reached out to Tonia Calderon and she had no comment.