He or she is buying Silver on the basis that it has underperformed over the years compared to gold and is due for a rise, the Silver to Gold ratio is prettty high, in the 20th century the average ratio was 47:1, it's currently 89:1.
But let's have a look at a Gold vs Silver performance at the current exchange rates for the AUD price over the last 20 years.
Gold up 515% over 20 years vs Silver up 303% over 20 years, it shows Silver has underperformed, but both have been still pretty good for a safer investment, with returns averaging @ 25% pa and 15% pa over the past 20 years currently on the Aussie price.
Inflation added over the past 20 years according to RBA is 70.3%, so both precious metals have done their job in beating inflation.
