My intent was to explore the new countries in World Adventures, but I was derailed when I learned about the "Irreversible Transfigurification" feature of the XL display case.  Put simply, if you fill all eight "slots" on the display case, you can then click on it and choose the action "Irreversible Transfigurification."  It costs $500, and you will never get back the items you put on the display case, so use this option with caution!

If you put at least five different CUTS of gems on the case, it will return you a crystal skull.  If you put at least five different TYPES of gems on the case, it will return you a giant crystal.  If you put metal ingots on, it will give back a bar of Compendium.  And apparently if you put a bunch of the broken stuff you find on your adventures up there, it will return you a Mysterious Mr. Gnome - although I have yet to test this out personally.

As always, there are a lot of caveats.  Most of the reports I have read state that the Irreversible Transfigurification goes by whichever item is in the top left slot, but I have not found this to be the case.  About one out of every four times, Irreversible Transfigurification seems to pick an item at random.  

For example, the last time I did it, I had a Septarian Nodule in the upper left slot, and a blue topaz to the right of that.  Sadly, I ended up with a blue topaz skull.  I say "sadly" because the Septarian Nodule alone was worth $520, the combined worth of all the gems on the display case was $1545, and the resulting blue topaz skull was worth a mere $530.  Lame!

From my experience, making Compendium ingots is your best financial bet.  I have yet to lose money making Compendium, even taking into account the $500 Transfigurification fee.  Start by loading up the display case with smelted ingots.  You will end up with a bar of Compendium which is worth more than the sum of the whole.  

If you put the bar of Compendium in the top left slot, and load up the rest of the case with more ingots, it will create an even more expensive bar of Compendium.  One user on the official Sims 3 forums reported having made a bar of Compendium which was worth several million dollars this way!

The last time I made a bar of Compendium I used ingots that were worth $2866, plus the $500 Transfigurification fee makes the lot worth $3366.  I ended up with a bar of Compendium worth $3582.  However, my calculations do not take into account the cost of having smelted all of those bits of ore.  Suffice it to say that the margins are slim, and Compendium may actually be a losing proposition if you take into account all of the overhead.

One benefit to concentrating your family's wealth into gold bars, super large crystals, and crystal skulls, is that the wealth can then be passed to other Sims.  Because all of these items can be placed into a Sim's personal inventory, you could theoretically take a lot of wealth with you when you move out.  (I haven't tested this yet.)