It is near vermilon.
you have to trade for it. the end of Digletts tunnel you see first has a house near it. once you know cut you can get in to the house, i think the man asks for an Abra in exchange for Mr Mime.
rock tunnel
other side of diglett tunnel
You take rock tunnel..........
in digletts tunnel any day between 12:00 and 16:00
head west of cerulean city to the Rock Tunnel
Digletts are the only Pokemon in Diglett's cave but they appear so often that when you take your first step into the cave that there is a 7 out of nine chance a Diglett will appear
Rock Tunnel is located due east of Cerulean City, around the corner from the pokecenter.
in power plant, near entrance to rock tunnel, have to surf to it
Rock Slide is taught by a move tutor in Rock Tunnel.
use the move Flash (TM70)
The ruby is in the tunnel behind the team rockets on mt. ember