We want to pay special attention to the frames quality. It is one of the most essential factors when deciding which frame to buy.
The best material are titanium and acetate. Titanium is the lightest and most durable of all; it will not corrode, stretch, or bend. Frames made of titanium may serve you for decades. Titanium is also a hypoallergenic material. Most titanium frames come with nose pads. You need to decide if you like nose pads or not.
As oppose to titanium, regular metal frames are subject to corrosion and easy to bend. Your glasses will change their shape almost instantly. This is usually the cheapest choice in retail or online stores. It costs several times less, but you get what you pay for. Adjustment of such frames is a horrible burden for the owner. We would definitely advise not to buy those frames.
Acetate is a premium material. Acetate is based on cotton and tree pulp cellulose. It is also hypoallergenic. In modern world, acetates are well known for being very strong, lightweight, and flexible. Cellulose acetate also has the widest range for transparency, rich colors, and finishes. Acetate frames tend to slightly melt from your body heat and self-adjust. This property makes them super comfortable to wear.
Injection mold frames are made by injecting a more brittle petroleum based plastic that is liquefied and then injected into an aluminum or stainless steel mold. Colors then added through a spray application, coated and tumbled for shine. Injection mold frames are generally less expensive but considered lower quality.