Diode forward voltage
As far as I know, the forward voltage of a diode depends on:


the diode’s technology (germanium diodes vs silicon diodes, PN junction diodes vs Shottky diodes),
the diode’s geometry (the larger the junction is, the smaller the forward voltage, everything else being equal),
the junction temperature,
the forward current.


As for “your” RS1BL diode, you can find the \$I_F\$ vs \$V_F\$ characteristic in the datasheet. You’ll see that \$V_F\$ should be close to 0.7V for low forward current (around 50mA).