I like how this works though I would suggest maybe a lisghtly different approach to tier 1. Instead of limiting its charges, why not make it place down explorer's wards instead? Or 2-minute wards? Still 4 charges, max out at any time 2.
Another option (that I personally like) would be to also make a "budget oracle's". Either oracle's extract or maybe a 3-minute CD item that grants 30 seconds of true sight or something. It could build out of Hunter's Machete.
I only suggest this because I feel that even these changes would keep mid-game warding far too easy to do with very little cost-effective counterplay available.
*highly supports cheaper early-sightstone with weaker base but upgrades more steadily*
Here was what I was thinking on the subject; actually:
Sightstone: 475g [colored white]
Ward Refresh: Can store up to 2 wards.
Ghost Ward: Can place up to 1 ward.
Ruby Sightstone: Sightstone[475g] + Ruby Crystal [475g] + Recipe  = 1300 [colored red]
Ward Refresh: Can store up to 4 wards.
Ghost Ward: Can have up to 2 wards.
Alchemist's Sightstone: Ruby Sightstone  + Philospher's Stone [700g] + Recipe [1000g] = 3000g [colored violet, also glowing]
+10 Health Regen per 5 seconds
+10 Mana Regen per 5 seconds
Transmute: +5 gold per 10 seconds.
Ward Refresh: Can store up to 8 wards.
Ghost Ward: Can have up to 4 wards.
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