Well in that case we're basing on luck, mostly.
Here's what you gotta do:
Get spire cut tiberiums, from 1 to 3, for each transfiguration;
Get 5 to 7 (depends on tiberium number above) of the gem you want as Spire Cut;
Make sure you aren't using more than 4 cut types;
Make sure you aren't using more than 4 different gems.
Notes: For higher chances to succeed, you need 3 Spire Cut Tiberiums (large or not, doesn't make a difference), and 5 of the other gem, in different cuts. Transfiguration works in a "the more of this, the higher the chance" system, so to make your chances to get a Spire Cut final result higher, you need to have either more spire cuts, or less of the other cuts. Same system applies to the gems, so you must make sure none of them have an unfair majority. You can use 1 Spire cut tiberium and 7 other gems, but that will lower the odds of you getting the Spire Cut.
The highest-chance setup, although requiring more Tiberiums, is something like this:
(I'm using Pink Diamond as an example. The result I want is a Large Spire Cut Pink Diamond)
Slot 1 (top left): Spire Cut Tiberium
Slot 2: Spire Cut Tiberium
Slot 3: Spire Cut Tiberium
Slot 4 (top right): Emerald Cut Rainbow Gem*
Slot 5 (bottom left): Heart-Shaped Pink Diamond
Slot 6: Heart-Shaped Pink Diamond
Slot 7: Brilliant Cut Pink Diamond**
Slot 8 (bottom right): Brilliant Cut Pink Diamond**
*The gem used in slot 4 is a random gem, but since it also has a small chance of being the final result, I used a gem that I also wanted as Spire Cut (Rainbow Gem), just to make sure. It's cut, however, must be different from all others.
**Slot 7 and 8's Pink Diamonds aren't Heart-Shaped because then the number of Heart-Shaped would be higher than the Spire Cut's, giving it majority and changing the final result to Heart-Shaped. We don't want that, so we change the cut of two of the Pink Diamonds to something else (this "something else" must be different from the cut used in Slot 4).
The setup above gives majority to Spire Cut and Pink Diamond, so the game should read this and pop out a Large Spire Cut Pink Diamond (You always get Large cuts when you transfigure gems into Spire Cut).
PS: Sorry if it sounded too complicated. Also, sorry for yet another huge post. On a sidenote, I didn't have time to test this, but I will after lunch (christmas lunch, yay), so if you don't want to risk your gems you may want to wait a little longer.