Who told you that the gold prices are stable? Have you checked the historical prices of gold? Check this:
http://www.nma.org/pdf/gold/his_gold_prices.pdf
The price of gold has increased from $19 per oz to $1,240 per oz in the 1900-2016 time period.
Even if you take the recent history, gold has given very good returns. The prices have increased from $271 per oz in 2001, to $1,240 per oz in 2016.
Gold is an immutable constant; it is to economics what the speed of light is to physics.
If the price appears to change, that is due to a shift in your frame of reference.