This question was inspired by the question about what constitutes a bardic spellcasting focus. It has been somewhat overcome by Tasha's Cauldron of Everything (p. 57) which explicitly describes a Ranger's Spell focus as a druidic focus, but since not all tables will use that optional rule the question will remain up.
A Ranger's spellcasting ability is described thusly:
By the time you reach 2nd level, you have learned to use the magical essence of nature to cast spells, much as a druid does.
As discussed here, a Ranger meets the definition of a spellcaster in that a class feature, not an item, grants the character the ability to cast spells. While that would support a Ranger using a spellcasting focus, only three spellcasting foci get explicit treatment in the rules (unless I missed a spot).
- A Holy Symbol (Basic Rules p. 48.)
- A Druidic Focus (Basic Rules p. 48.)
- An Arcane Focus (Basic Rules p. 47.)
Aside: I may be making a leap. The text on druidic focus does not mention Rangers. The leap is based on the Ranger class spellcasting feature being explicitly tied to druidic spellcasting in the brief text covering that class feature.
- Spellcasting Focus:
You can use a druidic focus (see “Equipment as a spellcasting focus for your druid spells).
- Druidic Focus.
A druidic focus might be a sprig of mistletoe or holly, a wand or scepter made of yew or another special wood, a staff drawn whole out of a living tree, or a totem object incorporating feathers, fur, bones, and teeth from sacred animals. A druid can use such an object as a spellcasting focus.
In a case of specific over general, Paladins and Clerics are called out as being able to use a holy symbol as a spellcasting focus (Rangers were left out). This left me with two questions, only one of which is germane to the Ranger2. The Ranger gets no specific call out (unlike the Paladin) on whether the character can use for a spellcasting focus a holy symbol, a druidic focus, or an arcane focus. (Since the Ranger is a spellcaster, there's got to be something that the class can use to substitute for material components, right?)
Why would this matter?
A lot of Ranger spells have material components. Just reviewing the SRD (which isn't the whole list) the following spells calling out "V,S,M" in the components line:
Alarm, Animal Friendship, Detect poison and disease, Goodberry, Jump, Longstrider
Animal Messenger, Barkskin, Darkvision, Locate Animals or Plants, Locate Object, Pass without a trace, Spike Growth
Nondetection, Water Breathing, Water Walk, Wind Wall
Conjure Woodland Beings, Freedom of Movement, Locate Creature, Stoneskin
Commune with Nature
The adventuring Ranger won't always be in a "home" environment
Considering that terrain types generally coincide with Druid Circles ...
Arctic, Coast, Desert, Forest, Grassland, Mountain, Swamp (I'd call seaborne adventure close enough to Coast for these purposes)
... it might be impossible to find mistletoe in the desert when a material component is needed.
Can the Ranger use an arcane focus or a holy symbol as a spellcasting focus, or is the ranger constrained to the druidic focus (a holly symbol1?) based on the class' spell casting feature being heavily druidic in nature1?
1 Please excuse the puns. Sometimes we can't help ourselves.
2 I leave the idea of druids being priest of nature (mostly gotten around with Nature Domain clerics) for another question regarding a holy symbol as a spellcasting focus for a druid.