Amethyst glass
Made with Manganese dioxide (MnO2). A transparent color. Some versions are prone to devitrification.
Same element does double duty: enough manganese gives a purple/violet glass, but small amounts are also used as a decolorizer to cancel out the green tint that iron impurities cause in "clear" glass. Some specific commercial purples are documented as notably prone to devitrification with repeated reheating. This varies by manufacturer, not by manganese content alone.
Why every color also needs to physically match
Chemistry sets a glass's color; physics decides whether it can actually be combined with other colors. Every glass has a Coefficient of Expansion (COE): how much it expands when heated and contracts when it cools. The number is in units of 10⁻⁷ per °C: COE 104 means 104 × 10⁻⁷ per °C, which is 10.4 parts per million per °C. If two colors in the same piece have different COEs, they pull against each other as the whole thing cools after being worked, building up internal stress. That stress shows up as cracking, sometimes immediately and sometimes weeks later, and it happens regardless of how well the colors chemically get along. This is separate from a color's chemistry: a compatible COE match doesn't guarantee a nice color combination, and a great color combination is worthless if the COEs don't match. Common named systems include COE 104 (soft lampworking glass, with long working time and the most color variety), COE 96 and COE 90 (fusing systems), and COE 33 (borosilicate, high-temperature). The specific number for any given colored rod depends on its manufacturer's formulation, not the colorant family alone, so this app doesn't assign COE numbers per color for that reason, and neither should you without checking the actual supplier data for the glass in hand.