While it seems that the answer to this question is obvious, it's actually a little more complex. In any gemstone, the color comes from the mineral composition of the stone before it forms (ex. sapphire is the mineral corundum) under intense heat and pressure. Genuine stones are formed in the earth, which leaves the levels of heat and pressure to chance, and can result in inconsistent color saturations or imperfections. Created stones are the same mineral composition as genuine stones, but they are formed in a laboratory where heat and pressure are consistent. This results in more predictability of color and fewer inclusions. Although lab created stones may appear more consistent and at times have nicer color than their genuine counterparts, they are often far less expensive because they are not created in nature. In reality, both genuine stones and lab created stones are like twins--they share the same mineral composition but may appear slightly different. 

Platinum used for jewelry is considered a pure metal, as it quite hard and does not need an additional alloy metal to make it strong enough for wear. Gold is an extremely soft metal, so much so that you could easily bend pure gold with your fingers. Gold must be blended with small amounts of other stronger metals (i.e. copper, nickel) to make it sturdy enough for everyday wear. The higher karat weight (10k, 14k, 18k) of your jewelry piece the more gold it contains. This difference in the hardness of the metals translates to price when creating a jewelry piece.  For example, to create a jewelry piece in 14k gold the designer needs 57.5%  gold precious metal and 42.5% alloy; to create a jewelry piece in platinum you would need 100% in platinum precious metal. More precious metal translates to a more expensive piece of jewelry.  See the photo below for a side by side comparison of gold vs. platinum cost at today's metal market prices.