Quantum3D were made up of a group of people who split off from 3dfx just after the Voodoo 1 came out. They created the first Voodoo SLI card out of these which was called the Obsidian 100SB.
They've subsequently produced a variety of unique designs based around 3dfx chipset. Whereas most companies were happy to licence the "reference" design from 3dfx, which is a safe route since it ensures maximum hardware and driver compatibility, Quantum3D were more adventurous.
They made an AGP Voodoo2 design for example which, in its day, made perfect sense since the Voodoo2 was the fastest 3D card of the time.
They also produced single board Voodoo2 SLI cards. The consumer version of this was the Obsidian2 X-24. This had a daughtercard which led to some overheating issues unless you fitted a fan blowing along the length of the card (indeed the final version from Quantum3D came fitted with one).
The professional version of the single board Voodoo2 SLI was the Obsidian2 200SBi which, not having the daughterboard is about a foot long.
This card was amazingly assembled in clusters of four producing 8 way Voodoo2 SLI! This was called the Heavy Metal Mercury and was used in military and industrial simulators. It's anti-aliasing capabilities were far ahead of its time. Indeed it was sneakily used by 3Dfx at a computer show to demonstrate the anti-aliasing capabilities of the Voodoo 5 5000 before the silicon had been finalised for it!
Quantum3D lost interest in the gaming/consumer end of the market possibly due to the support issues generated by overheating X-24s. Which is a shame since many of us love their characteristic black PCBs and are happy to fit the necessary fans.
They went on to make cards with four and eight way SLI VSA 100 chipsets (Voodoo4 and 5 chips) with the name Aalchemy. These were and are used for professional applications.
More lately they have used nVidia chips.
That's a hurricane tour. patience can probably provide you with a complete list of the cards and their specs. Hope this has been some help.