Pearlescent (Before General Knoxx)

Pearlescent is a controversial purported rarity whereby White coloured weapons are produced with special manufacturer titles. They include red text quotes and do have the special properties. Some have otherwise low-quality statistics of white weapons of the appropriate level, but some of them are powerful. Pearlescent weapons are a confirmed rarity weapon type that will be encountered in the third DLC, The Secret Armory of General Knoxx, so it is likely they were intended to exist. However, it is still not clear whether the purported Pearlescent weapons in the original release of Borderlands are true Pearlescents. It is also possible that their inclusion in The Secret Armory is merely a nod to the immense speculation and argument about the original "pearlescent" weapons. The Cyclops, Thanatos, and Invader have become Orange as a result of the latest patch. The actual rarity color of pearlescent weapons is light blue.

"Pearlescent" facts.

 * Prior to The Secret Armory of General Knoxx glitched white weapons that had similiar abilities and stats to orange weapons where believed to be "pearlescent" in rarity
 * Pearlescent weapons have white names and are sorted by rarity above dark orange in the inventory
 * Unique weapons, those obtained in the same way every time (quest reward, boss drop, specific chest) all have red text but are standard rarities (Green/Blue/Purple/Orange).
 * Each manufacturer that makes a sub-type of weapon makes exactly one randomly generated weapon of that type with red text.
 * These random weapons with red text are the only oranges.
 * These random weapons with red text are always orange with five exceptions (ogre is extremely rare):
 * Hyperion Invader (Pistol) Pearlescent pistol
 * S&S Thanatos Pearlescent pistol
 * Atlas Cyclops Pearlescent sniper ( can also be Dark Orange )
 * Dahl Penetrator Green/Blue/Purple sniper
 * Atlas Ogre Pearlescent combat rifle ( can also be Dark Orange ) can be aquired in Pearlescent when having a scope above 1.4x

Initial Theories
Many theorize that the controversial naming of Pearlescent guns is a product of one of three developer errors:

The most commonly accepted theory is that Pearlescent weapons (and the Penetrator) were always intended to be of Orange quality, but their names were accidentally given the same color as basic White weapons. This is supported by the small number of Pearlescent weapons, and the way they break the pattern formed by the other 45 Oranges. Also, Pearlescent weapons are rarely superior to Dark Orange, or even Yellow weapons of comparable level. Like all guns their power varies greatly based on their parts, it is unclear if they are as strong as normal oranges in general. The existence of weak Pearlescent guns leads to a second theory.

The second theory was that Pearlescent weapons were always intended to be of White quality, but because they have the signature red flavor text and special attributes, the inventory screen automatically bumps them up to the top of the list by default, despite the fact that they are not actually superior to Dark Orange weapons. This theory is discredited, however, by the unique Green, Blue and Purple weapons that have red text and special attributes but still appear below Yellows weapons on the inventory screen, accurately displaying their overall quality.

The third and perhaps most plausible theory is based on the game's random weapons generator as explained here: http://borderlandsguide.com/forum/read-pearl-rarity-glitch-read. The generator assembles guns using a component-based system, then tallies a "score" based on the quality and power of each component. This score is ranked from 3 to 100, with Dark Orange legendaries having a total score of 67 to 100. Because some components seem to mistakenly have assigned values that are inappropriately high, the score of some weapons exceeds 100, causing the game to rank it as a White item while having the nomenclature of a legendary item. This theory lends credence to the second theory as most red text abilities are linked to the erroneous gun components in question.

In any case, the title of ‘Pearlescent’ as a genuinely higher quality tier is defunct, and they would be considered either as equivalent to Oranges or as White weapons of slightly higher quality, respectively. That the game's programming only recognizes seven rarity levels and excludes Pearlescent confirms it is a glitch.

Rarity Level
Weapons have a hidden stat called Rarity Level that determines their color. It can be seen on PC using the debug console. Rarity Level is the sum of rarity scores for all weapon parts.

Manufacturer random weapons come with two fixed weapon parts. One of them is a special Title part that starts with "Title.TitleM_", and has only cosmetic affects on gameplay. It determines the weapon name, adds rarity, and adds the red text to the Item Card. The second part is the one that actually affects stats and adds the red text effect to the weapon. It also affects rarity, but usually not as much:

The rarest Purple weapons generated are about Rarity Level 30. Adding 50 Rarity with a Title part puts a weapon right at the minimum Rarity Level to be Light Orange. The quality of the other parts might upgrade it to Orange or Dark Orange. Thus the rarest Dark Orange are around 80. The Pearlescents got 50 rarity added twice and so are always above 100.