Diamonds are more reliable because the price of gold constantly changes. It depends on how well gold is doing in the market and the grade of the diamond. Artificial diamonds are very cheap. Gold prices change often. The price change in 1980 is an example of this.
An ounce of gold runs about $1390.00 but an ounce of industrial use diamonds is about 141carats and can sell from $1-$5/ carat which would be $705/ounce at the $5.00 rate, but that same 141 carats of jewelry grade diamond can sell for $50 a carat=$7050/ounce. Like a car diamonds lose value the minute they leave the store but jewelry grade diamonds are still worth more per ounce. these are not retail prices
It depends on the size of the diamond "stones". A single diamond weighing an ounce would be far more valuable than golds. If you have an ounce of diamonds dust, it would be worth a small fraction of the price of gold.