Security of rand in ruby compared to other methods

What's the security difference between:

rand(10**100).to_s(16)
# => "a8ef61cbac3b770580cdebb55c1d2cf65cf32b5df57ef44a3bea4658ff66ac37f93b540bfb4c2ddc33e"

And RandomSecure and all the others? is rand(...).to_s(...) secure enough?