What makes surfaces portal-able?

The portal-conductive surfaces are generally concrete slabs (for the majority of Portal) or white panels (for the majority of Portal 2). They are easily identified by their more pale colour, whereas the darker, more metallic, tin or wooden surfaces are not portal-conductive.

There's not a canon-explanation for what makes a particular surface conductive, it's simply that the above listed surfaces are so.

In addition to concrete and white panels, Moon Gel (made from crushed moon rocks) is a gel capable of making a particular surface portal conductive, which is explained in some of Cave Johnson's dialogue:

"The bean counters told me we literally could not afford to buy seven dollars worth of moon rocks, much less seventy million. Bought 'em anyway. Ground 'em up, mixed em into a gel."

"And guess what? Ground up moon rocks are pure poison. I am deathly ill."

"Still, it turns out they're a great portal conductor. So now we're gonna see if jumping in and out of these new portals can somehow leech the lunar poison out of a man's bloodstream. When life gives you lemons, make lemonade. [coughs] Let's all stay positive and do some science."

Throughout Portal 1 and 2, we have observed several surfaces to be capable of carrying portals. Besides the white test chamber panels which are obviously designed for the purpose, concrete has often been sufficient to carry a portal, and we're also informed that moon dust is an excellent portal conductor.

My hypothesis is that portals are best conducted by silicon oxide compounds (particularly aluminium-silicon-oxide, which is the primary form found on the moon's surface). Concrete certainly falls into this category, as it is essentially manufactured from terrestrial rock which, like most rock, is composed of silicon oxide compounds. Other materials such as wood and metal contain almost no silicon and cannot conduct portals. Metal seems to be often employed when a strictly non-portal surface is required, perhaps suggesting that electrically-conductive metal is able to earth / short out portal energy to ensure that a portal cannot form.