Regarding stock and non-liquid capital, I've previously written:
See this website for an interesting attempt to define what types of income are deductible and not deductible for Ma'aser purposes.
While I see some deductions and non-deductions on his lists that I question, the overall effort is admirable, and it makes a great starting point for this discussion.
As a side note, a rabbi of mine once said that he believed (and Rav Dovid at the above website seems to agree with this, though I don't know the source for either of them) that you do not need to pay Ma'aser on money until it has been realized to you. In other words, taxes taken out of your paycheck before the money is received by you do not need to be considered as part of your income (ie, are deductible) for purposes of calculating Ma'aser. On the flip side of that (this is my own extrapolation, not the words of my rabbi), if you receive a tax refund, you would have to pay Ma'aser on the refunded money - if you did not previously take Ma'aser on the taxes that were removed from your paycheck.
Since capital gains and appreciation are not of any benefit to you until you liquidate the assets, I would think that you do not need to pay Ma'aser on them, especially if you are talking about the assessed value of real property. However, if you receive dividends or gains that you decide to immediately roll over into the purchase of new stock, etc., you might have to pay Ma'aser on those gains prior to the rollover. But definitely, DEFINITELY do not take my word on that, or any of what I said above in my own name. I'm not a Rav or Posek. Ask your own local Orthodox rabbi for further guidance and especially for Psak.
I don't see how this is any different than any other non-liquid assets or capital appreciation. It has no inherent value other than what can be acquired with it. So, unless and until you liquidate it - or transfer it into some usable form, such as exchange for actual goods and services - there is nothing from which to tithe.
As in the above, Tzarich 'Iyun and CYLOR.