Some devotees worship Govardhan shilas as expanded energies of Krishna in His lila of playing on and around, as well as lifting Govardhan Hill. See below for references that Srila Vishvanath Chakravarti Thakur uses to describe the various kinds of shilas found at Govardhan Hill and Who or what the various colourations identify Them as being.
Shri Vriti Chintamani by Shrila Vishvanatha Chakravarti Thakur.
......the chapter of Govardhana hill will describe the nectar about the Radha, Balarama, and Shyama Shilas.
kvacic chila nilamani-pravinah
kutrapi ta marakataika-tanah
tah padmaragah sphatikac ca kaccid
govardhan adrau vidhu-keli-kalpah
The rocks of Govardhana Hill are all precious gems. Some are sapphires, some emeralds, some rubies or sphatika crystals. They provide the perfect backdrop for Lord Shri Krishna's pastimes.
krsnagrajo 'ram vrsabhanuja dhih
duram prayati sphatikoccayantat
krsnagrajo 'trety api sapi yatra
If, while walking on Govardhana Hill, Krishna's elder brother Balarama sees a series of golden stones, He thinks: "....the daughter of King Vrsabhanu must have just now walked before Me," and He changes course to avoid an awkward situation. In the same way, if She sees a series of crystal stones, She thinks: "...Balarama must have just now walked before Me," and She changes Her course.
krsnas tu jambunada-madhyam eti
If, on Govardhana Hill, which is the best of Lord Hari's servants, Sri Krishna sees a golden stone, He takes it to be His beloved. In the same way, His beloved sees a sapphire (or a blackish blue stone), She assumes it is Her lover.
yasyagga-cobha na vilobhayanti
kam va nikuYjesu daricu drstah
krsnasya kanta-nivahasya hara-
The gopis' necklaces, ornaments, red foot cosmetic, betelnuts, and other paraphernalia gaze on the beauty of Govardhana Hill's groves and caves. Who would not become enchanted by the beauty of Govardhana Hill?