A CS2 Doppler phase is the version of the Doppler finish you actually received. Valve does not print the phase on the item. The pattern index picks it at the moment the item is created, and the community sorted every possible outcome into named groups.
For regular Doppler, the phases are Phase 1, Phase 2, Phase 3 and Phase 4, plus the gems Ruby, Sapphire and Black Pearl. For Gamma Doppler, the phases are again 1 to 4, and the single gem is Emerald.
What does each Doppler phase look like?
Phase 1 mixes dark red and blue on a black base. Phase 2 leans pink and purple with a lot of red. Phase 3 is mostly black and dark green with faint blue. Phase 4 shows strong blue with little red. Ruby is almost fully red, Sapphire almost fully blue, and Black Pearl is deep black with purple shimmer. Emerald, on Gamma Doppler, is a solid bright green.
These descriptions are enough for a rough guess, but the safe way to confirm is to run the inspect link through a float checker, which reads the pattern index and names the phase.
Which Doppler phase is worth the most?
The gems cost the most because fewer pattern indexes land on them. Ruby and Sapphire usually lead, Black Pearl sits near them, and Emerald leads Gamma Doppler. Among the numbered phases, Phase 2 and Phase 4 tend to fetch a small premium over Phase 1 and Phase 3, though this shifts with demand.
Phase changes nothing about the weapon in play. The value gap is purely about looks and scarcity. That gap is also why phase is a common target for mislabelled listings, so treat any gem-priced item that has not been verified by tool as unconfirmed.