The vital thing needed for producing optimal vocal tone is correct resonance and placement. Without these the voice will never sound consistent or at it's best. Every time I sing I immediately "place" my voice in it's spot. This might sound nebulous, but it's all about sensations so it's hard to show or demonstrate. Imagery is the best way to help singers find their optimum placement and resonance, along with a fair amount of experimentation.
Physically my most resonant position is behind the sinuses and above the hard palate. I like to think of two knitting needles driven into the middle of my skull. If one was located through the bridge of my nose, and the other through the temples, where they intersect is about where my ideal placement resides. You can manipulate this placement of the voice easily, and that's why it's hard to get it right. Try pushing forward into the nose and you will feel a very frontal, thin placement. Try taking it back and you will experience a hollow, throaty sound that has almost no resonance. Move between the two until a blended sound is achieved. You want the hard, nasal resonance mixed with the hollow, spacey sound to achieve a fully rounded, resonant vocal tone. Eventually, with the help of your singing teacher who will give feedback as to when the sound is at it's best, you'll lock in your placement and it'll become a powerful muscle memory, and therefore easy to replicate at will.
Think of an apple. It has a core and a round body. Your voice should have the same.