It's common sense

Let's say you were single when you bought a house for 100K by cash no mortgage, and two years later when you got married the house worth 120K, of course the 120K is the pre-marital property.   The original price is irrelevant.  

Let's say you where single when you bought stock worth 100K, and two years later the stock is worth 120K when you got married, of course the 120K is the premarital property, the original price is irrelevant. . 

Get it? 

The stock example is straight forward.  For the house example, the complicating part is the mortgage.  This is where "equity" is useful term.  Equity is the value you had in the house, it is the market value of the house minus debt.  Your downpayment is part of the equity, the monthly principal payment is part of the equity, the appreciation is part of the equity.  So when you get married, all you need to do is to find out how much equity you have in the house, i.e.  market value - mortgage balance = equity.  This equity amount are not subject to divorce division.