Zinc is one of the impurities that must be removed from gold in the refining process. It is highly volatile. As a mineral, it is also very bitter tasting. Many of us have zinc (bitterness, wormwood) in our hearts. Someone comes along with acid behavior, and we explode in his face! There are many tomatoes out there, whom God uses to refine the zinc out of our gold.

For every zinc problem, there is a corresponding tomato. That is a spiritual law. The dross must be removed before our hearts can be refined by the baptism of fire into pure gold. It will not leave our hearts by itself. It must first manifest itself, crystallize, or bubble to the surface before Jesus removes it out of our lives.

Lev. 19:17 and 18 says,
17 'You shall not hate your fellow countryman in your heart; you may surely reprove your neighbor, but shall not incur sin because of him. 18 'You shall not take vengeance, nor bear any grudge against the sons of your people, but you shall love your neighbor as yourself; I am the LORD.

Ac 3:19
Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord; 20 And he shall send Jesus Christ, which before was preached unto you: 21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.